Method for producing metal by molten salt electrolysis and apparatus used for the production method

ABSTRACT

Provided is a method for obtaining a particular metal at high purity, with safety, and at low cost, from a treatment object containing two or more metal elements. The present invention provides a method for producing a metal by molten salt electrolysis, the method including a step of dissolving, in a molten salt, a metal element contained in a treatment object containing two or more metal elements; and a step of depositing or alloying a particular metal present in the molten salt, on one of a pair of electrode members disposed in the molten salt containing the dissolved metal element, by controlling a potential of the electrode members to a predetermined value.

TECHNICAL FIELD

The present invention relates to a method for producing a metal bymolten salt electrolysis; and an apparatus used for the productionmethod.

BACKGROUND ART

Known methods of smelting ores to provide particular metals arepyrometallurgy and hydrometallurgy.

The pyrometallurgical smelting is a method of melting an ore in a hightemperature furnace to separate a target metal. For example, aconcentrate, a roasted ore, or a sintered ore is melted in a hightemperature furnace, concentrated into a crude metal ingot while gangue,impurities, and the like are separated as slag (Non Patent Literature(NPL) 1, p. 46).

In smelting, since the specific gravity difference between molten metalsis utilized to separate the metals from an ore, the specific gravitydifference between the metals to be separated needs to be large. Inaddition, separation targets need to have low solubility in each other.Since elements that satisfy such conditions between metal materials arelimited, target elements separated by pyrometallurgy are limited, whichis problematic.

The hydrometallurgy is a method of dissolving an ore in, for example, analkaline or acidic solution and separating and extracting a target metalfrom the solution. A method for separating and extracting the targetmetal from this aqueous solution is, for example, a method using ionexchange, a method using solvent extraction, or a method using aqueoussolution electrolysis.

In the method using ion exchange, a solid substance that partially has agroup of ions allowing ion exchange and is referred to as an ionexchanger is used to perform reversible ion exchange (NPL 1, p. 194).

Ion exchange, which uses the adsorption capability and exchangecapability of an ion-exchange resin, is an excellent treatment. However,since this treatment is performed by repeating of adsorption anddissociation of ions, ion exchange is not suitable for economically andefficiently treating a large amount of substance, which is problematic.

The method using solvent extraction is a separation method using thedifference in the distributions of different solutes in two solventsthat are immiscible with each other (NPL 1, p. 199).

In this solvent extraction, for example, an acid treatment is performedto achieve ionization; and, in separation, a large number of treatmentprocesses need to be performed. In these processes, large amounts ofacid and alkali are required and a large amount of wastewater isgenerated, which is problematic.

In the method of electrolysis smelting using aqueous solutionelectrolysis, the presence of a difference between elements in tendencyfor anode dissolution or cathode deposition is used and a pure metal isproduced. Simultaneously, in the electrolytic solution used, reactionsof generating slightly soluble salts from impurity ions are also used(NPL 1, p. 219).

However, metal elements that can be separated and deposited bypurification using aqueous solution electrolysis are limited. Forexample, deposition of rare earth materials cannot be theoreticallyachieved, which is problematic.

Regarding Al, electrolytic smelting utilizing molten salt electrolysisis also known. In this method, three layers of Al (purification targetmaterial) alloyed so as to have a decreased melting point, a moltensalt, and the recovered metal, are formed and the specific gravitydifference is utilized to perform smelting. Since the specific gravitydifference is thus utilized, smelting needs to be performed while allthe three layers are melted (NPL 1, p. 254).

The target metal of this method is Al. In addition, when the potentialof an impurity present with the purification target metal is close tothe potential of the purification target metal, entry of the impurityinto the deposited target metal occurs, which is problematic.

On the other hand, a method for recovering tungsten is described in, forexample, NPL 2 as follows.

Hard scrap or soft scrap of cemented carbide tools is made to react withsodium nitrate molten salt and then dissolved in water to produce anaqueous solution of sodium tungstate. The aqueous solution of sodiumtungstate is treated by an ion-exchange method using an ion-exchangeresin to produce an aqueous solution of ammonium tungstate. From theaqueous solution of ammonium tungstate, ammonium paratungstate (APT) iscrystallized. After that, the thus-crystallized ammonium paratungstateis calcined, reduced, and carbonized to provide tungsten carbide.

The hard scrap collectively denotes pieces of scrap still having theshapes of products. The soft scrap denotes powder-form scrap such aspowder dust and cutting dust generated during processing for producingcemented carbide tools.

Patent Literature (PTL) 1 proposes, in the production of sodiumtungstate by oxidizing hard alloy scrap and/or heavy metal scrap in amolten salt bath, use of a molten salt containing 60 to 90 wt % ofsodium hydroxide and 10 to 40 wt % of sodium sulfate. PTL 1 alsoproposes that the reaction between such scrap and the molten salt isperformed in a rotary kiln that is operated as batch processes and canbe directly heated.

However, in the above-described method described in NPL 2, the reactionbetween hard scrap or soft scrap of cemented carbide tools and sodiumnitrate molten salt occurs very vigorously. Accordingly, the reaction isdifficult to control and the operation has safety problems. In addition,when hard scrap or soft scrap of cemented carbide tools is made to reactwith sodium nitrate molten salt, metals contained in the hard scrap orsoft scrap of cemented carbide tools, such as vanadium and chromium,take the form of water-soluble metal oxide ions and enter the aqueoussolution of sodium tungstate. As a result, because of the presence ofsuch metals as impurities, high purity is difficult to achieve, which isproblematic.

In the above-described method described in PTL 1, sodium sulfate moltensalt serving as an oxidizing agent has a high melting point of 884° C.Accordingly, the temperature during reaction needs to be set to a hightemperature of 884° C. or more. As a result, metal materials arecorroded, which is problematic. In addition, the reaction proceedsslowly and hence the reaction is time-consuming and involves a largeenergy loss, which is problematic.

On the other hand, lithium is mainly extracted from lithium-containingores (such as spodumene, amblygonite, petalite, and lepidolite), andsalt lakes and underground brine that have high lithium concentrations.However, Japan does not have lithium-containing ores or salt lakes.Accordingly, almost the whole amount of lithium is actually supplied byimports.

Thus, recently, studies were started regarding separation and recoveryof lithium from, for example, lithium-containing waste generated in theproduction steps of lithium-containing products such as lithiumbatteries or waste of used lithium-containing products.

The following method for recovering lithium has been proposed: lithiumcobalt oxide serving as the positive electrode material of lithiumsecondary batteries, together with metallic lithium, is subjected to areduction reaction in lithium chloride molten salt, so that lithiumoxide is generated and cobalt or cobalt oxide is separated byprecipitation; after that, lithium oxide is electrolyzed in the lithiumchloride molten salt, so that metallic lithium is deposited on thecathode and recovered (PTL 2: Japanese Unexamined Patent ApplicationPublication No. 2005-011698).

However, in this method, in order to separate cobalt contained in thetreatment object by reduction, addition of metallic lithium is required.The method employs the step of adding metallic lithium in order torecover metallic lithium and hence is inefficient, which is problematic.

A method for recovering lithium has been proposed in which a mixture ofcarbon and lithium manganese oxide serving as the positive electrodematerial of lithium secondary batteries is roasted in any one of the airatmosphere, an oxidizing atmosphere, an inert atmosphere, and a reducingatmosphere, to turn the lithium into lithium oxide; and this roastedsubstance is immersed in water so that lithium is leached in the form oflithium hydroxide and lithium carbonate (PTL 3).

However, in this method, since lithium hydroxide and lithium carbonatedo not have high solubility, the recovery efficiency is low. Inaddition, a large amount of water is required to leach lithium hydroxideand lithium carbonate into water and, as a result of the treatment, alarge amount of wastewater is generated, which is problematic.

Furthermore, tantalum (Ta) is mainly used in tantalum capacitors and canbe recovered from tantalum capacitor scrap. Specifically, tantalum isrecovered by processes of oxidation treatment, magnetic separation,screening, separation with running water, grinding, screening, leaching,oxidation treatment, reduction treatment, and leaching (refer to NPL 3,pages 319 to 326).

Vanadium (V) is used as an additive to steel or a desulfurizationcatalyst in oil refining. Vanadium used as an additive to steel iscollected in the form of steel scrap and recycled as steel. Spentcatalysts can be sequentially subjected to steps of classification,roasting, grinding, leaching, filtration, leaching solution,dehydration, thermal decomposition, and melting, so that vanadiumpentoxide can be obtained (NPL 3, pages 391 to 396).

Molybdenum (Mo) is also used as an additive to steel, alloy, or adesulfurization catalyst in oil refining. Molybdenum used as an additiveto steel or an alloy element is collected in the form of steel or alloyand used, without being extracted, in the form of steel or alloy. Spentcatalysts can be sequentially subjected to steps of roasting, removal ofoil, water, and sulfur, leaching in basic condition, and recovery, sothat Mo can be obtained (NPL 3, pages 301 to 303).

Niobium (Nb) is mainly used as an additive to steel. Niobium used as anadditive to steel is collected in the form of steel scrap. However, theniobium content of high-tensile steel, stainless steel, and the like isvery low and niobium itself is not recycled (NPL 3, page 339).

Manganese (Mn) is mostly used for steel and aluminum alloy and collectedin the form of steel scrap and aluminum alloy scrap, respectively. Inthe case of recycling of steel, a high proportion of manganese is leftin various slags and such manganese forming slags is not suitable forrecycling. Manganese in slag is partially used in, for example, amanganese-calcium silicate fertilizer.

Aluminum cans containing such aluminum alloy are collected and thenrecycled (NPL 3, pages 343 to 344).

Chromium (Cr) used for steel (stainless steel) and superalloy iscollected in the form of steel scrap and superalloy scrap, respectively,and then recycled; and extraction and recovery of elemental chromium isnot performed (NPL 3, pages 219 to 221).

In the above-described recycling techniques, recovery involves a largenumber of processes such as roasting (heating), grinding, leaching, andreduction. And the processes are complicated and hence the treatment istime-consuming and costly, which is problematic.

In addition, the treatment requires roasting and, in the treatment,substances that are not the extraction target are also treated,resulting in unnecessary energy consumption. Furthermore, by subjectingsubstances that are not the treatment target to the roasting treatment,unnecessary oxides are generated, resulting in a large amount of waste.In addition, since acid treatment or base treatment is performed, thetreatment produces acid or base wastewater, which exerts a load on theenvironment.

In summary, existing metal recycling techniques have problems asfollows: for example, the treatment is costly, energy loss is large, theamount of waste is large, and the environmental load is heavy. Inaddition, because of problems in terms of cost or technique, some metalsare not recycled as simple substances.

CITATION LIST Non Patent Literature

-   PTL 1: Japanese Unexamined Patent Application Publication    (Translation of PCT Application) No. 11-505801-   PTL 2: Japanese Unexamined Patent Application Publication No.    2005-011698-   PTL 3: Japanese Unexamined Patent Application Publication No.    2011-094227-   NPL 1: Courses of Contemporary Metallurgy, Smelting Part, Vol. 2,    Nonferrous Metal Smelting, edited by The Japan Institute of Metals    and Materials (1980), pages 46, 194, 199, 219, and 254-   NPL 2: Rare-Metal High-Efficiency-Recovery-System Development    Project “Recovery of tungsten etc. from waste cemented carbide    tools”, Metal Resource Report, Vol. 38, No. 4, November 2008, pp.    407-413-   NPL 3: Compilation of Noble Metal and Rare Metal Recycling    Techniques, published by NTS Inc., planned and edited by Bookers    Ltd., the first impression of the first edition, Oct. 19, 2007

SUMMARY OF INVENTION Technical Problem

In view of the above-described problems, an object of the presentinvention is to provide a method for producing a metal, the method beingapplicable to any ore and providing high purity metal at low cost; andan apparatus used for the production method. An object of the presentinvention is to provide a method for producing a metal, the methodproviding a particular metal at high purity, with safety, and at lowcost, from a treatment object containing two or more metal elements; andan apparatus used for the production method.

Solution to Problem

An embodiment of the present invention is a method for producing a metalby molten salt electrolysis, the method including a step of dissolving,in a molten salt, a metal element contained in a treatment objectcontaining two or more metal elements; and a step of depositing oralloying a particular metal present in the molten salt, on one of a pairof electrode members disposed in the molten salt containing thedissolved metal element, by controlling a potential of the electrodemembers to a predetermined value.

In another embodiment of the present invention, the treatment object isan ore or a crude metal ingot obtained from the ore.

Another embodiment of the present invention is a method for producingtungsten, wherein a metal element contained in the treatment object istungsten, in the step of dissolving, in a molten salt, a metal elementfrom a treatment object, tungsten is dissolved from the treatmentobject, and in the step of depositing or alloying a particular metal,tungsten present in the molten salt is deposited on one of a pair ofelectrode members disposed in the molten salt containing dissolvedtungsten, by controlling a potential of the electrode members to apredetermined value.

In another embodiment of the present invention, the treatment object isa metal material containing the tungsten.

In another embodiment of the present invention, the treatment object isa metal material containing tungsten and a transition metal.

In another embodiment of the present invention, the treatment object isa cemented carbide product.

Another embodiment of the present invention is a method for producinglithium, wherein a metal element contained in the treatment object islithium, in the step of dissolving, in a molten salt, a metal elementfrom a treatment object, lithium is dissolved from the treatment object,and in the step of depositing or alloying a particular metal, lithiumpresent in the molten salt is deposited on one of a pair of electrodemembers disposed in the molten salt containing dissolved lithium, bycontrolling a potential of the electrode members to a predeterminedvalue.

In another embodiment of the present invention, the treatment object isa material containing lithium and a transition metal.

In another embodiment of the present invention, the treatment object isa battery electrode material containing lithium.

In another embodiment of the present invention, the treatment objectcontains a transition metal or a rare earth metal.

In another embodiment of the present invention, the treatment objectcontains one or more metals selected from the group consisting of V, Nb,Mo, Ti, Ta, Zr, and Hf.

In another embodiment of the present invention, the treatment objectcontains Sr and/or Ba.

In another embodiment of the present invention, the treatment objectcontains one or more metals selected from the group consisting of Zn,Cd, Ga, In, Ge, Sn, Pb, Sb, and Bi.

In another embodiment of the present invention, the molten salt isselected such that, in the step of depositing or alloying a particularmetal, a difference between a standard electrode potential of a simplesubstance or alloy of the particular metal and a standard electrodepotential of a simple substance or alloy of another metal in the moltensalt is 0.05 V or more.

In another embodiment of the present invention, in the step ofdepositing or alloying a particular metal, the potential of theelectrode members is controlled to the predetermined value so that theparticular metal element in the molten salt is selectively deposited oralloyed.

In another embodiment of the present invention, in the step ofdissolving, in a molten salt, a metal element contained in a treatmentobject, the metal is dissolved in the molten salt by a chemicalprocedure.

In another embodiment of the present invention, in the step ofdissolving, in a molten salt, a metal element contained in a treatmentobject, a cathode and an anode that is formed of an anode materialcontaining the treatment object are disposed in the molten salt, and apotential at the anode is controlled to a predetermined value so that ametal element corresponding to the controlled potential is dissolved inthe molten salt from the treatment object.

In another embodiment of the present invention, the molten salt isselected such that, in the step of dissolving, in a molten salt, a metalelement contained in a treatment object, a difference between a standardelectrode potential of a simple substance or alloy of the particularmetal and a standard electrode potential of a simple substance or alloyof another metal in the molten salt is 0.05 V or more.

In another embodiment of the present invention, in the step ofdissolving, in a molten salt, a metal element contained in a treatmentobject, the potential at the anode is controlled to a predeterminedvalue so that the particular metal element is selectively dissolved inthe molten salt.

In another embodiment of the present invention, in the step ofdissolving, in a molten salt, a metal element contained in a treatmentobject, one or more metals each serving as the particular metal aredissolved in the molten salt.

In another embodiment of the present invention, the particular metaldeposited or alloyed is a transition metal.

In another embodiment of the present invention, the particular metaldeposited or alloyed is a rare earth metal.

In another embodiment of the present invention, the particular metaldeposited or alloyed is V, Nb, Mo, Ti, Ta, Zr, or Hf.

In another embodiment of the present invention, the particular metaldeposited or alloyed is Sr or Ba.

In another embodiment of the present invention, the particular metaldeposited or alloyed is Zn, Cd, Ga, In, Ge, Sn, Pb, Sb, or Bi.

In another embodiment of the present invention, the molten salt is achloride or fluoride molten salt.

In another embodiment of the present invention, the molten salt is amolten salt mixture containing a chloride molten salt and a fluoridemolten salt.

In another embodiment of the present invention, the treatment object hasa form of particles or powder.

In another embodiment of the present invention, the treatment objecthaving the form of particles or powder is compacted to form the anode.

Another embodiment of the present invention is a method for producing ametal by molten salt electrolysis, the method being a method forproducing a particular metal by molten salt electrolysis from atreatment object containing two or more metal elements, wherein acathode and an anode that is formed of an anode material containing thetreatment object are disposed in a molten salt, and a potential at theanode is controlled to a predetermined value so that a metal elementcorresponding to the controlled potential is dissolved in the moltensalt from the treatment object and a particular metal is left in theanode.

In another embodiment of the present invention, the treatment object isan ore or a crude metal ingot obtained from the ore.

Another embodiment of the present invention is a method for producingtungsten by molten salt electrolysis from a treatment object containingtungsten, wherein a cathode and an anode that is formed of an anodematerial containing the treatment object are disposed in a molten salt,and a potential at the anode is controlled to a predetermined value sothat a metal element corresponding to the controlled potential isdissolved in the molten salt from the treatment object and tungsten isleft in the anode.

In another embodiment of the present invention, the molten salt isselected such that, in the step of dissolving a metal element in themolten salt from the treatment object, a difference between a standardelectrode potential of a simple substance or alloy of the particularmetal and a standard electrode potential of a simple substance or alloyof another metal in the molten salt is 0.05 V or more.

Another embodiment of the present invention is an apparatus used for amethod for producing a metal by molten salt electrolysis, the apparatusincluding a container containing a molten salt; a cathode immersed inthe molten salt contained within the container; and an anode that isimmersed in the molten salt contained within the container and thatcontains a treatment object containing two or more metal elements,wherein the molten salt is movable into and out of the anode, theapparatus further includes a control unit configured to control apotential of the cathode and the anode to a predetermined value, and avalue of the potential is changeable in the control unit.

Another embodiment of the present invention is an apparatus used for amethod for producing a metal by molten salt electrolysis, the apparatusincluding a container containing a molten salt containing two or moredissolved metal elements; a cathode and an anode that are immersed inthe molten salt contained within the container; and a control unitconfigured to control a potential of the cathode and the anode to apredetermined value, wherein a value of the potential is changeable inthe control unit.

In another embodiment of the present invention, the two or more metalelements include at least one of tungsten and lithium.

Advantageous Effects of Invention

A method for producing a metal and an apparatus used for the productionmethod according to the present invention are applicable to any ore. Useof a production method or an apparatus used for the production methodaccording to the present invention can provide a particular metal athigh purity, with safety, and at low cost, from a treatment objectcontaining two or more metal elements.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart for explaining an embodiment of the presentinvention.

FIG. 2 is a schematic view describing examples of deposition potentialsof rare earth metals in a molten salt.

FIG. 3 is a graph illustrating examples of a relationship betweentreatment time and concentration of ions of a rare earth metal in amolten salt in an embodiment of the present invention.

FIG. 4 is a schematic sectional view for explaining the configuration ofan apparatus according to an embodiment of the present invention.

FIG. 5 is a schematic sectional view for explaining the configuration ofan apparatus according to an embodiment of the present invention.

FIG. 6 is a flow chart for explaining another embodiment of the presentinvention.

FIG. 7 is a schematic sectional view for explaining another embodimentof the present invention.

FIG. 8 is a schematic sectional view for explaining another embodimentof the present invention.

FIG. 9 is a schematic sectional view for explaining another embodimentof the present invention.

FIG. 10 is a schematic sectional view for explaining another embodimentof the present invention.

FIG. 11 is a schematic sectional view for explaining a modification ofanother embodiment of the present invention.

FIG. 12 is a schematic sectional view for explaining a modification ofanother embodiment of the present invention.

FIG. 13 is a schematic sectional view for explaining a modification ofanother embodiment of the present invention.

FIG. 14 is a photograph for explaining an anode electrode used inexamples according to the present invention.

FIG. 15 is a graph illustrating the relationship between the value ofanode current and time in an example according to the present invention.

FIG. 16 is a scanning electron micrograph of a surface portion of acathode electrode used in an electrolysis step in an example accordingto the present invention. The scale in the lower right of the micrographindicates a length of 8 μm.

FIG. 17 is a scanning electron micrograph illustrating Dy distributionstatus in the regions of the micrograph illustrated in FIG. 16.

FIG. 18 is a schematic sectional view for explaining an example of theconfiguration of an apparatus according to an embodiment of the presentinvention.

FIG. 19 is a schematic sectional view for explaining an example of theconfiguration of an apparatus according to an embodiment of the presentinvention.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention is a method for producing a metalby molten salt electrolysis, the method including a step of dissolving,in a molten salt, a metal element contained in a treatment objectcontaining two or more metal elements; and a step of depositing oralloying a particular metal present in the molten salt, on one of a pairof electrode members disposed in the molten salt containing thedissolved metal element, by controlling a potential of the electrodemembers to a predetermined value.

First Embodiment

In the first embodiment, the treatment object is an ore containing twoor more metal elements or a crude metal ingot obtained from the ore(hereafter sometimes simply referred to as a crude metal ingot).

That is, roughly speaking, this embodiment includes a process ofdissolving, in a molten salt, a metal contained in an object (the ore orcrude metal ingot), and a process of depositing a metal or an alloy of aseparation-extraction target element on one of electrodes (cathode) froma molten salt containing the dissolved metal by molten saltelectrolysis. A feature of this embodiment is that, by controlling thepotential of the electrodes, a particular target element is selectivelydissolved or deposited to achieve separation and smelting.

The process of dissolving, in a molten salt, a metal element containedin an object will be first described.

A procedure for dissolving, in a molten salt, a metal element containedin an ore or a crude metal ingot is, for example, a chemical procedurefor dissolution. Specifically, an ore or a crude metal ingot is groundinto particles or powder, mixed with a salt, and heated. As a result,two or more metal elements contained in the ore or the crude metal ingotcan be dissolved in the molten salt. Alternatively, the treatment objectmay be placed in a molten salt and dissolved.

Another procedure is an electrochemical procedure. Specifically, anobject is disposed as an anode in a molten salt and the value of thepotential at the object is controlled to selectively dissolve an elementcontained in the object: molten salt electrolysis is characterized inthat different elements are dissolved at different potentials; and suchcharacteristics are utilized to selectively separate metalscorresponding to potentials. In this way, by using an object as an anodeand controlling the potential during dissolution, a metal element thatis a smelting target can be selectively dissolved in a molten salt.

In the process of dissolving, in a molten salt, a metal elementcontained in an object, the potential is preferably controlled such thatimpurities contained in the object remain undissolved. As a result,entry of impurities in the subsequent deposition process can be reduced.

For this purpose, the molten salt is preferably selected such that, inthe step of dissolving, in the molten salt, a metal element contained inan ore or a crude metal ingot, the difference between the standardpotential of a simple substance or alloy of a particular metal (metalelement to be dissolved) and the standard electrode potential of asimple substance or alloy of another metal in the molten salt is 0.05 Vor more. As a result, the metal element that is dissolved in the moltensalt can be sufficiently separated from the metal element that is leftin the anode. The difference between the standard electrode potentialsis more preferably 0.1 V or more, still more preferably 0.25 V or more.

The value of the potential controlled at the anode can be calculated byNernst equation described below.

In the case where a plurality of target particular metals are containedin an ore or a crude metal ingot used, the potential may be controlledsuch that respective metals are dissolved in a molten salt.Alternatively, after one of the particular metals is dissolved, the oreor crude metal ingot (anode) containing the remainder of the metals maybe moved to another molten salt and the potential may be similarlycontrolled to a predetermined value so that the remainder of theparticular metals is dissolved in the molten salt.

Some metals are more easily separated by deposition described below. Insuch cases, the entire treatment object may be dissolved or only aparticular metal and some other metals may be dissolved.

From the standpoint of reduction of entry of impurities, in the step ofdissolving, in a molten salt, a metal element contained in an ore or acrude metal ingot, the potential at the anode is preferably controlledto a predetermined value so that the particular metal element isselectively dissolved in the molten salt.

The molten salt can be selected from chlorides and fluorides. Examplesof chloride molten salts include KCl, NaCl, CaCl₂, LiCl, RbCl, CsCl,SrCl₂, BaCl₂, and MgCl₂. Examples of fluoride molten salts include LiF,NaF, KF, RbF, CsF, MgF₂, CaF₂, SrF₂, and BaF₂. In the cases where rareearth elements are subjected to molten salt electrolysis, chloridemolten salts are preferably used in view of efficiency; in particular,KCl, NaCl, and CaCl₂ are preferably used because they are inexpensiveand easily available.

Among such molten salts, a plurality of molten salts can be combined andused as a molten salt having a desirable composition. For example, amolten salt having a composition such as KCl—CaCl₂, LiCl—KCl, orNaCl—KCl may be used.

The cathode is formed of carbon or a material that tends to form analloy with an alkali metal such as Li or Na constituting cations in themolten salt. For example, aluminum (Al), zinc (Zn), gallium (Ga),cadmium (Cd), indium (In), tin (Sn), antimony (Sb), lead (Pb), orbismuth (Bi) may be used.

When the ore or crude metal ingot is used as an anode, for example, theore or crude metal ingot contained in a conductive basket formed of ametal or the like may be disposed in the molten salt. An opening may beformed in an upper portion of the basket so that the ore or crude metalingot serving as the treatment object can be inserted through theopening into the basket; and a large number of holes may be formed inthe side and bottom walls of the basket so that the molten salt can flowinto the basket. The basket may be constituted by a desired materialsuch as a mesh member knitted from metal wires or a sheet member that isa sheet-shaped metal plate having a large number of holes. Inparticular, it is effective that the material is formed of C, Pt, Mo, orthe like.

In the cases where the object is an ore or the like and has a highelectric resistance, the contact area between the object and theconductive material is preferably increased. The object is effectivelyused as an electrode by, for example, wrapping the object with a metalmesh member or filling the object into spaces within a metal porousmember.

When the cathode and the basket containing an ore or crude metal ingotare disposed in the molten salt and the potential at the anode (basket)is controlled from the outside as described above, a target metal can bedissolved in the molten salt from the ore or crude metal ingot.

In the subsequent deposition process, molten salt electrolysis isperformed with a pair of electrode members disposed in the molten saltso that a metal element dissolved in the molten salt is deposited on oneof the electrode members (cathode). In this case, by controlling thepotential value in the molten salt electrolysis, a particular metalelement can be selectively deposited as metal or alloy on the cathode.

As in the dissolution process, in this deposition process, molten saltelectrolysis is characterized in that different elements are depositedat different potentials as metal or alloy on the cathode; and suchcharacteristics are utilized to separate the metals. Thus, even when aplurality of target particular metals are contained in the molten salt,by controlling the potential, the metals can be individually depositedon cathodes one by one.

The electrode members may be formed of, for example, nickel (Ni),molybdenum (Mo), or glassy carbon (C).

In this embodiment, the above-described two processes are used toseparate and extract from an object a particular metal element that is asmelting target. In this embodiment, since a molten salt is used, thesystem needs to be heated such that the temperature of the system in theprocesses is equal to or more than the melting point of the molten salt.

A feature of the two processes is use of a molten salt. Thus, the factthat different molten salts have different dissolution-depositionpotentials for elements is utilized and the processes can be designed byselecting a molten salt such that the dissolution-deposition potentialsof a particular metal element that is a target element and the otherimpurity metal elements are values that allow easy performance of theprocesses. Specifically, the molten salt is preferably selected suchthat, in the step of depositing or alloying the particular metal, thedifference between the standard electrode potential of a simplesubstance or alloy of the particular metal and the standard electrodepotential of a simple substance or alloy of another metal in the moltensalt is 0.05 V or more. In the molten salt, the difference between thestandard electrode potential of a simple substance or alloy of theparticular metal and the standard electrode potential of a simplesubstance or alloy of another metal is more preferably 0.1 V or more,still more preferably 0.25 V or more.

In this way, in the step of depositing or alloying the particular metal,the potential of the electrode members is preferably controlled to apredetermined value so that the particular metal element in the moltensalt is selectively deposited or alloyed.

The deposition potential of a simple substance or alloy of a metal to bedeposited on the cathode can be determined by electrochemicalcalculation. Specifically, the calculation is performed with Nernstequation.

For example, the potential at which a simple substance of praseodymium(Pr) is deposited from trivalent Pr ions (hereafter represented byPr(III)) can be determined with the following equation.

E _(Pr) =E ⁰ _(Pr) +RT/3F·ln(a _(Pr(III)) /a _(Pr(0)))  Equation (1)

In Eq. (1), E⁰ _(Pr) represents the standard potential, R represents thegas constant, T represents absolute temperature, F represents theFaraday constant, a_(Pr(III)) represents the activity of Pr(III) ions,and a_(Pr(0)) represents the activity of Pr simple substance.

When Eq. (1) is rewritten in view of activity coefficient γ_(Pr(III)),since a_(Pr(0))=1, the following equations are provided.

$\begin{matrix}\begin{matrix}{E_{\Pr} = {E_{\Pr}^{0} + {{{RT}/3}\; {F \cdot \ln}\; a_{\Pr {({III})}}}}} \\{= {E_{\Pr}^{0} + {{{RT}/3}\; {F \cdot {\ln \left( {\gamma_{\Pr {({III})}} \cdot C_{\Pr {({III})}}} \right)}}}}}\end{matrix} & {{Equation}\mspace{14mu} (2)} \\{E_{\Pr} = {E_{\Pr}^{0^{\prime}} + {{{RT}/3}\; {F \cdot \ln}\; C_{\Pr {({III})}}}}} & {{Equation}\mspace{14mu} (3)}\end{matrix}$

In Eq. (3), C_(Pr(III)) represents the concentration of trivalent Prions, and E^(0′) _(Pr) represents formal electrode potential (here,equal to E⁰ _(Pr)+RT/3F·ln γ_(Pr(III))).

Similarly, the potential at which PrNi alloy is deposited on theelectrode surface (deposition potential: E_(Pr.Ni)) can be determinedwith the following equation.

E _(Pr.Ni) =E ^(0′) _(Pr.Ni) RT/3F·ln C _(Pr(III))  Equation (4)

In Eq. (4), E^(0′) _(Pr.Ni) represents formal electrode potential (here,equal to E⁰ _(Pr.Ni)+RT/3F·ln γ_(Pr(III))).

Similarly, by using the above-described equations, deposition potentialsof all deposits corresponding to different molten salts can bedetermined. In the process of depositing or alloying a particular metalon the cathode, in view of the deposition potential value of thisparticular metal or an alloy thereof, a deposit that provides apotential difference with respect to another metal or an alloy thereofis selected or the order of depositions is determined.

Voltage and current during operation vary depending on the size orpositional relationship of electrodes. Accordingly, reference values ofvoltage and current are determined on the basis of conditions andsubsequently voltage and current are determined in each step on thebasis of the potential value and order determined by the above-describedmethod.

As described above, in a method for producing a metal by molten saltelectrolysis according to this embodiment, the potential value iscontrolled to thereby electrochemically dissolve and deposit a targetmetal. Accordingly, the steps can be simplified, compared with, forexample, the existing wet treatment involving repeating of processes ofdissolution and extraction using acid or the like; and a particularelement can be selectively separated and recovered. In addition,adjustment of the specific gravity of the molten salt is not necessary;and, by selecting a low-temperature molten salt in which an object canbe treated in the solid state, a simple apparatus configuration can beemployed. Moreover, the operation pattern can also be simplified. As aresult, the steps can be performed efficiently at low cost.

Alternatively, a particular metal can be smelted on the basis of an ideathat is totally contrary to the above-described idea of depositing oralloying a particular metal on the cathode.

That is, a method for producing a metal according to this embodiment isa method for producing a particular metal by molten salt electrolysisfrom an ore containing two or more metal elements or a crude metal ingotobtained from the ore, wherein a cathode and an anode that is formed ofan anode material containing the ore or crude metal ingot are disposedin a molten salt, and the potential at the anode is controlled to apredetermined value so that a metal element corresponding to thepotential is dissolved in the molten salt from the ore or crude metalingot and a particular metal is left in the anode.

In this method, the object (the ore or crude metal ingot) is used as theanode and metal elements other than a particular metal element, that is,only metal elements serving as impurities are dissolved in the moltensalt, so that the particular metal is left in the anode. In this case,by also controlling the potential at the anode, such a phenomenon iscaused in which the metal element that is the smelting target is left inthe anode and impurity elements are dissolved in the molten salt. As aresult, a smelted metal material is obtained in the anode.

In this method, the molten salt is also preferably selected such that,in the step of dissolving, in the molten salt, a metal element from theore or crude metal ingot, the difference between the standard electrodepotential of a simple substance or alloy of the particular metal and thestandard electrode potential of a simple substance or alloy of anothermetal in the molten salt is 0.05 V or more. As a result, the particularmetal can be sufficiently separated from the other metal and theparticular metal alone can be left in the anode. The difference betweenthe standard electrode potentials is more preferably 0.1 V or more,still more preferably 0.25 V or more.

The value of the potential controlled at the anode can be calculated byNernst equation as described above.

Ores usable in the method for producing a metal by molten saltelectrolysis according to this embodiment are ores containing targetparticular metals. Examples of the ores include gold ore, silver ore,copper ore, iron ore, aluminum ore, lead ore, zinc ore, tin ore, mercuryore, sulfur ore, phosphorus ore, nickel ore, cobalt ore, manganese ore,chromium ore, molybdenum ore, tungsten ore, antimony ore, arsenic ore,bismuth ore, strontium ore, beryllium ore, magnesium ore, barium ore,and calcium ore. For example, rare earth metals can be obtained frombastnaesite, monazite, loparite, apatite, xenotime, fergusonite, andeudialyte.

The crude metal ingot obtained from the ore denotes a metal containing atarget particular metal at a low purity, such as a metal obtained bysmelting the ore.

The method for producing a metal by molten salt electrolysis accordingto this embodiment is suitably applied to an ore or crude metal ingotobtained from the ore that is used as the anode and contains atransition metal or a rare earth metal.

The transition metal is not particularly limited and may be any elementamong from group 3 (group IIIA) to group 11 (group IB) of the periodictable. The rare earth metal is also not particularly limited and may beany element among scandium (Sc), yttrium (Y), and 15 lanthanoid elementsin group 3 (group IIIA) of the periodic table.

The method for producing a metal by molten salt electrolysis accordingto this embodiment is also suitably applicable to the cases whereparticular metals deposited or alloyed on cathodes are rare earthmetals. In this embodiment, appropriate selection of the composition ofthe molten salt allows even deposition of rare earth metals that cannotbe deposited by aqueous solution electrolysis. Thus, rare earth metalsthat are difficult to mine as resources can be easily obtained.

In this embodiment, the ore or crude metal ingot obtained from the orepreferably has the form of particles or powder. When the ore or crudemetal ingot to be treated is prepared so as to have the form ofparticles or powder, the surface area is increased and the treatmentefficiency can be increased. From this viewpoint, the maximum particlesize of the ore or crude metal ingot is preferably 0.01 mm to 2 mm, morepreferably 0.01 mm to 1 mm, still more preferably 0.01 mm to 0.2 mm.

In addition, the ore or crude metal ingot in the form of particles orpowder is preferably compacted to form the anode. The ore or crude metalingot in the form of powder can be compacted and, as a result, can beused as the anode. In this case, between the particles, there aredesirably spaces that the molten salt can easily enter.

Hereinafter, this embodiment will be described with reference todrawings. In the drawings below, the same or corresponding parts aredesignated by the same reference signs and repetitive descriptionsthereof are omitted.

First Embodiment-1

An example of this embodiment will be described that is a method forobtaining neodymium (Nd), dysprosium (Dy), and praseodymium (Pr) bymolten salt electrolysis from an ore containing Nd, Dy, and Pr. Examplesof the ore include monazite, apatite, xenotime, fergusonite, andeudialyte.

As illustrated in FIG. 1, a preparation step (S10) is first performed.

In this step, for example, an ore that is a treatment object, a moltensalt to be used, and an apparatus including, for example, electrodes anda container for containing the molten salt are prepared. Optionally, inorder to promote dissolution of the treatment object in the molten salt,the treatment object may be finely ground for the purpose of increasingthe contact area between the treatment object and the molten salt.

The ore containing Nd, Dy, and Pr may be, for example, xenotime ore. Forexample, a xenotime ore has a composition of 3.0% Nd, 7.9% Dy, and 0.5%Pr.

Subsequently, a dissolution step in the molten salt (S20) is performed.

In this step (S20), the ore and (another) electrode member are immersedin the prepared molten salt; the ore and the electrode member areconnected via a power supply so that the potential of the ore and theelectrode member is controlled. By controlling the potential at the ore,rare earth elements (Nd, Dy, and Pr) in the ore are selectivelydissolved in the molten salt. The molten salt used may be a molten salthaving a desired composition.

For example, the molten salt may be LiF—NaF—KF; the other electrodemember may be an electrode formed of glassy carbon; and theabove-described ore may be used as the treatment object.

In this case, for example, while the molten salt is heated at 700° C.,Nd, Dy, and Pr can be selectively dissolved in the molten salt from theore. The potential is controlled to a value at which elements other thanNd, Dy, and Pr are scarcely dissolved in the molten salt but Nd, Dy, andPr are dissolved in the molten salt.

Subsequently, as illustrated in FIG. 1, a separation extraction step(S30) is performed.

Specifically, in the molten salt in which Nd, Dy, and Pr are dissolvedas described above, a pair of electrodes are inserted and the potentialof the electrode members is controlled to a predetermined value. Forexample, in the case of using LiCl—KCl molten salt, as illustrated inFIG. 2, the potential value is controlled to potentials corresponding todeposition potentials determined for respective rare earth metals. As aresult, by controlling the potential, the rare earth metal deposited onthe electrode can be selected. Thus, the rare earth metals can beselectively recovered element by element.

For example, as illustrated in FIG. 2, rare earth elements such as Nd,Dy, and Pr have different deposition potential values for respectiveelements. Specifically, as illustrated in FIG. 2, the depositionpotential of Nd is about 0.40 V (vs. Li⁺/Li); the deposition potentialsof Pr and Dy are about 0.47 V (vs. Li⁺/Li); and the deposition potentialof DyNi₂, which is a Dy compound, is about 0.77 V (vs. Li⁺/Li).

The deposition potentials in FIG. 2 are described with reference to Li.In FIG. 2, the ordinate axis indicates deposition potential (unit: V).Such deposition potentials are values in the case where the molten saltis LiCl—KCl and the temperature of the molten salt is set at 450° C.

As described above, elements and compounds have different depositionpotentials. Accordingly, by immersing a pair of electrodes in a moltensalt in which particular metals are dissolved and by controlling thepotential at the cathode so as to correspond to the above-describeddeposition potentials, particular rare earth elements can be selectivelydeposited on the cathode. By changing the potential value at the cathode(for example, sequential potential changes), particular metals to bedeposited can be selected.

For example, as illustrated in FIG. 3, different voltages aresequentially applied across a pair of electrodes immersed in the moltensalt in which Nd, Dy, and Pr are dissolved. The concentrations (ionconcentrations) of Nd, Dy, and Pr in the molten salt are each 0.5 mol %.

When data described in FIG. 2 are used as deposition potential values,for example, LiCl—KCl is used as the molten salt and the temperature ofthe molten salt is set at 450° C. In FIG. 3, the abscissa axis indicatestreatment time and the ordinate axis indicates the ion concentrations ofrare earth elements in the molten salt. The unit in the ordinate axis ismol %.

In STEP 1, when Ni is first used as a cathode material and the potentialat the cathode is set to a value that is less noble than 0.77 V (vs.Li+/Li) and slightly more noble than 0.63 V (vs. Li⁺/Li) (for example,the potential difference is set to 0.631 V (vs. Li⁺/Li)), Dy ions arealloyed with the cathode material Ni so that DyNi₂ is deposited on thecathode surface. As a result, as illustrated in FIG. 3, the Dy ionconcentration in the molten salt is sharply decreased. Dy can berecovered until the Dy ion concentration in the molten salt becomesabout 3.6×10⁻⁴ mol %.

Subsequently, in STEP 2, when another electrode (for example, a Moelectrode) is used as a cathode and the potential at the cathode is setto a value that is slightly more noble than 0.40 V (vs. Li⁺/Li) (forexample, the potential difference is set to 0.401 V (vs. Li⁺/Li)), Pr isdeposited on one of the electrodes (cathode). As a result, asillustrated in FIG. 3, the Pr ion concentration in the molten salt issharply decreased. Pr can be recovered until the Pr ion concentration inthe molten salt becomes about 0.017 mol %.

The electrode used in STEP 2 is different from the electrode on whichDyNi₂ has been deposited in STEP 1. For example, the electrode on whichDyNi₂ has been deposited in STEP 1 may be removed from the molten saltbefore STEP 2 is started, and another electrode may be immersed in themolten salt; alternatively, the electrode on which DyNi₂ has beendeposited may be left unremoved and, in STEP 2, the potential at anotherelectrode may be controlled.

Subsequently, in STEP 3, when the potential at still another electrode(for example, a Mo electrode) is set to 0.10 V (vs. Li⁺/Li), Nd isdeposited on this electrode (cathode). As a result, as illustrated inFIG. 3, the Nd ion concentration in the molten salt is sharplydecreased. Nd can be recovered until the Nd ion concentration in themolten salt becomes, for example, about 2.7×10⁻⁷ mol %.

The electrode on which Pr has been deposited in STEP 2 may be removedfrom the molten salt before STEP 3 is started, and another electrode maybe immersed in the molten salt; alternatively, the electrode on which Prhas been deposited in STEP 2 may be left immersed in the molten saltand, in STEP 3, another electrode may be used.

DyNi₂ recovered in STEP 1 is treated in STEP 4: the electrode on thesurface of which DyNi₂ has been deposited and another electrode (forexample, a Mo electrode) are immersed in a molten salt; and thepotential at the DyNi₂ electrode is set to be in a potential range inwhich Dy is dissolved but Ni is not dissolved (0.77 or more and 2.6 orless V (vs. Li⁺/Li)), so that Dy can be dissolved in the molten salt andDy alone can be deposited on the surface of the other electrode.

As has been described above, the target particular metals can beindividually recovered from the molten salt.

Apparatus Used for Method of this Embodiment

Hereinafter, an apparatus used for the method of this embodiment in FIG.1 will be described with reference to FIGS. 4 and 5. A recoveryapparatus illustrated in FIG. 4 includes a container 1 containing amolten salt, a molten salt 2 contained within the container 1, a basket4 containing a treatment object (the ore or crude metal ingot) 3,electrodes 6 to 8, a heater 10 for heating the molten salt 2, and acontrol unit 9 electrically connected to the basket 4 and the electrodes6 to 8 via conductive wires 5. The control unit 9 is configured tocontrol the potential (change the potential) of one electrode that isthe basket 4 and the other electrode that is one of the electrodes 6 to8. In the control unit 9, the value to which the potential is controlledis changeable. The heater 10 is disposed so as to circularly surroundthe container 1. The electrodes 6 to 8 may be formed of desiredmaterials. For example, the electrode 6 may be formed of nickel (Ni).For example, the electrodes 7 and 8 may be formed of carbon (C). Thecontainer 1 may have a bottom surface that has a circular shape or apolygonal shape. The basket 4 may be the above-described basket.

The basket 4 and the electrodes 6 to 8 are controlled by the controlunit 9 to predetermined potential values. By controlling the electrodes6 to 8 to different potentials, different particular metalscorresponding to the controlled potential values are deposited on thesurfaces of the electrodes 6 to 8 as described below. For example, asdescribed below, the potential value set for the electrode 6 can beadjusted so that a DyNi₂ film 11 is deposited on the surface of theelectrode 6. By adjusting the potential set for the electrode 7, a Prfilm 12 can be deposited on the surface of the electrode 7. By adjustingthe potential set for the electrode 8, a Nd film 13 can be deposited onthe surface of the electrode 8.

The electrode 6 on which the DyNi₂ film 11 is deposited is then placedin a container 1 containing a molten salt 2 as illustrated in FIG. 5.Furthermore, another electrode is placed in the molten salt 2 so as toface the electrode 6 on the surface of which the DyNi₂ film 11 isdeposited. The electrodes 6 and 15 are connected to a control unit 9 viaconductive wires 5. While the molten salt 2 is heated with a heater 10disposed so as to surround the container 1, the control unit 9 is usedto control the potential of the electrodes 6 and 15 to a predeterminedvalue. At this time, the potential is controlled such that the potentialat the cathode (electrode 15) is the deposition potential of Dy.

As a result, from the DyNi₂ film 11 deposited on the surface of theelectrode 6, Dy is dissolved in the molten salt 2 and a Dy film 16 isdeposited on the surface of the electrode 15. The heating temperaturefor the molten salt 2 with the heater 10 may be, for example, 800° C. inboth of the treatments using the apparatuses illustrated in FIGS. 4 and5. In this way, particular metals can be deposited as simple substanceson the surfaces of the electrodes 7, 8, and 15.

In the case where the method of this embodiment is performed with theapparatuses illustrated in FIGS. 4 and 5, for example, the method may beperformed in the following manner.

The ore (9 kg) is first prepared as the treatment object 3 andLiF—NaF—KF is prepared as the molten salt 2. For example, the ore maycontain 3.0 wt % of Nd, 0.5 wt % of Pr, and 7.9 wt % of Dy. The ore isground and placed within the basket 4. From the viewpoint of enhancementof the treatment efficiency, the size of the ore serving as thetreatment object 3 is preferably minimized by grinding. For example, theore is ground into particles having a maximum particle size of 2 mm orless, preferably 1 mm or less, more preferably 0.2 mm or less. Theamount of the molten salt 2 is about 16 liters (mass: 25 kg).

The treatment object 3 contained in the basket 4 and one of theelectrodes 6 to 8 are used as a pair of electrodes and STEP 1 to STEP 3of the method of this embodiment described with reference to FIGS. 2 and3 are performed. Specifically, in the above-described STEP 1, thetreatment object 3 contained in the basket 4 and the electrode 6 areused as a pair of electrodes and the potential of the electrodes iscontrolled to a predetermined value. As a result, DyNi₂ is deposited onthe surface of the electrode 6. In the above-described STEP 2, thetreatment object 3 contained in the basket 4 and the electrode 7 areused as a pair of electrodes and the potential of the electrodes iscontrolled to a predetermined value. As a result, Pr is deposited on thesurface of the electrode 7. The mass of a Pr film deposited on thesurface of the electrode 7 in FIG. 4 is, for example, about 30 g toabout 50 g.

In the above-described STEP 3, the treatment object 3 contained in thebasket 4 and the electrode 8 are used as a pair of electrodes and thepotential of the electrodes is controlled to a predetermined value. As aresult, Nd is deposited on the surface of the electrode 8. The mass of aNd film deposited on the surface of the electrode 8 is, for example,about 200 g to about 300 g.

In the above-described STEP 4, the electrode 6 and the electrode 15 areplaced in the apparatus illustrated in FIG. 5 and the potential of theelectrodes in the molten salt is controlled to a predetermined value. Asa result, Dy is deposited on the surface of the electrode 15. The massof a Dy film 16 deposited on the surface of the electrode 15 is, forexample, 600 g to 800 g.

As described with reference to FIG. 4, the step of dissolving targetmetals in the molten salt 2 and the step of depositing particular metalsas simple substances on the surfaces of the electrodes 7, 8, and thelike can be performed within the same apparatus (with the same moltensalt 2). On the other hand, the step of separating and extracting Dyfrom DyNi₂ described above in STEP 4 is preferably performed in anapparatus (apparatus illustrated in FIG. 5) other than the apparatus(apparatus illustrated in FIG. 4) used for the step of dissolving metalsin the molten salt 2 described with reference to FIG. 4.

As has been described above, particular metals (for example, Dy, Pr, andNd can be recovered from an ore or crude metal ingot serving as thetreatment object 3.

First Embodiment-2

An example of this embodiment will be described that is a method forobtaining neodymium (Nd), dysprosium (Dy), and praseodymium (Pr) bymolten salt electrolysis from a crude metal ingot obtained by smeltingan ore containing Nd, Dy, and Pr.

The crude metal ingot containing Nd, Dy, and Pr may be, for example,mixed rare earth metal (didymium). A smelting method for obtaining themixed rare earth metal is not particularly limited and may be selectedfrom publicly known methods.

As illustrated in FIG. 6, a step (S11) of preparing a crude metal ingotserving as a treatment object is first performed. Specifically, asillustrated in FIG. 7, a crude metal ingot serving as a treatment object3 is immersed in a molten salt 2 contained within a container 1; and aconductive wire 5 is connected to the treatment object 3, the conductivewire 5 being used for connection to a power supply in a control unit 9.The salt used was LiCl—KCl.

In the molten salt 2, an electrode material 25 contained within a basket24 and serving as the other electrode is immersed together with thebasket 24. The electrode material 25 is a material that tends to form analloy with an alkali metal such as Li and Na constituting cations in themolten salt. Examples of the electrode material 25 include aluminum(Al), zinc (Zn), gallium (Ga), cadmium (Cd), indium (In), tin (Sn),antimony (Sb), lead (Pb), and bismuth (Bi).

Subsequently, as illustrated in FIG. 6, a step (S21) of dissolving Nd,Dy, and Pr in a molten salt is performed.

Specifically, as illustrated in FIG. 7, the potential of the treatmentobject 3 and the electrode material 25 contained within the basket 24 iscontrolled with the control unit 9, so that the potential at thetreatment object 3 is adjusted to a predetermined value. As a result,rare earth elements such as Nd, Dy, and Pr are dissolved in the moltensalt 2 from the crude metal ingot serving as the treatment object 3.

Subsequently, as illustrated in FIG. 6, a step (S31) of depositing DyNi₂by electrolysis is performed. Specifically, instead of the electrodematerial 25 contained in the basket 24 in FIG. 7, as illustrated in FIG.8, an electrode 6 formed of nickel is immersed in the molten salt 2.This electrode 6 is connected to the control unit 9 via a conductivewire 5. In this state, the control unit 9 is used to control thepotential of the treatment object 3 serving as one electrode and theelectrode 6 serving as the other electrode, to a predetermined value.

As a result, rare earth elements such as Dy are dissolved in the moltensalt 2 from the treatment object 3 and DyNi₂ is deposited on the surfaceof the electrode 6 from the molten salt 2.

Subsequently, as illustrated in FIG. 6, a step (S32) of recovering Pr byelectrolysis is performed. Specifically, as illustrated in FIG. 9,instead of the treatment object 3, an electrode 27 formed of carbon isimmersed as one electrode in the molten salt 2. In addition, instead ofthe electrode 6 in FIG. 8, an electrode 7 formed of carbon is placed ata position so as to face the electrode 27 and be immersed in the moltensalt 2. The electrode 27 and the electrode 7 are electrically connectedto the control unit 9 via conductive wires 5. In this state, thepotential of one electrode 27 and the other electrode 7 is controlled toa predetermined value.

As a result, Pr dissolved in the molten salt 2 is deposited on thesurface of the electrode 7. When a chloride is used as the molten salt2, chlorine gas (Cl₂) is released from the region around the electrode27.

Subsequently, as illustrated in FIG. 6, a step (S33) of recovering Nd byelectrolysis is performed. Specifically, instead of the electrode 7, asillustrated in FIG. 10, an electrode 8 formed of carbon is placed so asto face the electrode 27 and be immersed in the molten salt 2. Thiselectrode 8 is electrically connected to the control unit 9 via aconductive wire 5. The control unit 9 is used to control the potentialof the electrode 8 and the electrode 27 to a predetermined value. As aresult, Nd is deposited on the surface of the electrode 8. At this time,chlorine gas is released from the region around the electrode 27.

Subsequently, a step (S34) of recovering Dy by electrolysis from DyNi₂recovered in the step (S31) is performed. Specifically, as illustratedin FIG. 5, the electrode 6 on the surface of which DyNi₂ is deposited(refer to FIG. 8) is immersed in the molten salt 2; the other electrode15 is disposed so as to be immersed in the molten salt 2; and thecontrol unit 9 is used to control the potential of the electrodes 6 and15 to a predetermined value. As a result, Dy is temporarily dissolved inthe molten salt 2 from DyNi₂ deposited on the surface of the electrode 6and then a Dy film 16 is deposited on the surface of the electrode 15.Thus, Nd, Dy, and Pr, which are rare earth metals, can be individuallyrecovered.

The above-described steps (S21 to S32) may be performed with thefollowing apparatus configurations. For example, the above-describedstep (S31) may be performed with an apparatus configuration illustratedin FIG. 11.

Specifically, instead of the treatment object 3 in the apparatusconfiguration in FIG. 8, the basket 24 containing a material 26 alloyedby the step illustrated in FIG. 7 is immersed in the molten salt 2. Asillustrated in FIG. 11, this basket 24 is electrically connected to thecontrol unit 9 via a conductive wire 5. The potential of the electrode 6and the material 26 contained within the basket 24 and alloyed by thestep illustrated in FIG. 7 is controlled to a predetermined value. As aresult, Dy dissolved in the molten salt 2 is deposited as DyNi₂ on thesurface of the electrode 6. Dy can be recovered as a simple substancefrom DyNi₂ deposited on the surface of the electrode 6, by the same stepas the step (S34) in FIG. 6.

Subsequently, the above-described step (S32) may be performed by atreatment with an apparatus configuration illustrated in FIG. 12.Specifically, instead of the electrode 6 in FIG. 11, as illustrated inFIG. 12, an electrode 7 formed of carbon is placed at a position so asto face the basket 24 and be immersed in the molten salt 2. Thiselectrode 7 is electrically connected to the control unit 9 via aconductive wire 5. The control unit is used to control the potential ofthe electrode 7 and the alloy 26 contained within the basket 24, to apredetermined value. As a result, Pr dissolved in the molten salt 2 isdeposited on the surface of the electrode 7.

Subsequently, the above-described step (S33) may be performed by atreatment with an apparatus configuration illustrated in FIG. 13.Specifically, as illustrated in FIG. 13, instead of the electrode 7 inFIG. 12, an electrode 8 formed of carbon is placed at a position so asto face the basket 24 and be immersed in the molten salt 2. Theelectrode 8 is electrically connected to the control unit 9 via aconductive wire 5. The control unit 9 is used to control the potentialof the electrode 8 and the alloy 26 disposed within the basket 24, to apredetermined value. As a result, Nd is deposited on the surface of theelectrode 8.

By using the method having been described, particular metals containedin a crude metal ingot can be sequentially individually recovered.According to the method of this embodiment, the apparatus configurationcan be simplified and the treatment time can also be decreased, comparedwith the existing wet separation method and the like. Thus, the costincurred for obtaining elements such as rare earth elements can bereduced. In addition, by appropriately setting a potential at anelectrode, a particular metal can be deposited as a simple substance onthe electrode surface and hence high purity metal can be obtained. Thepotentials for depositing individual metals and alloys can be determinedby the above-described calculation.

Second Embodiment

A method for producing tungsten by molten salt electrolysis according tothis embodiment is a method for producing tungsten by molten saltelectrolysis from a treatment object containing tungsten, the methodincluding a step of dissolving, in a molten salt, tungsten from thetreatment object, and a step of depositing tungsten present in themolten salt, on one of a pair of electrode members disposed in themolten salt containing dissolved tungsten, by controlling a potential ofthe electrode members to a predetermined value.

That is, roughly speaking, this embodiment includes a process ofdissolving, in a molten salt, tungsten contained in the treatmentobject, and a process of depositing tungsten on one of electrodes(cathode) from the molten salt containing dissolved tungsten by moltensalt electrolysis. A feature of this embodiment is that, by controllingthe potential of the electrodes, tungsten is selectively deposited froma treatment object to provide high purity tungsten.

The process of dissolving, in a molten salt, tungsten contained in atreatment object will be first described.

A procedure for dissolving, in a molten salt, tungsten contained in atreatment object is, for example, a chemical procedure for dissolution.Specifically, a treatment object is ground into particles or powder,mixed with a salt, and heated. As a result, tungsten contained in thetreatment object can be dissolved in the molten salt. Alternatively, atreatment object may be placed in a molten salt and dissolved.

Another procedure is an electrochemical procedure. Specifically, ananode formed of an anode material containing a treatment object isplaced in a molten salt and the value of the potential at the treatmentobject placed as the anode is controlled to selectively dissolvetungsten contained in the treatment object. Molten salt electrolysis ischaracterized in that different elements are dissolved at differentpotentials. Such characteristics can be utilized to separate tungstenform other metals. In this way, by using a treatment object as an anodeand controlling the potential during dissolution, tungsten can beselectively dissolved in a molten salt.

In this step, the entire treatment object may be dissolved, or atungsten-containing portion of the treatment object or tungsten alonemay be dissolved. Conditions under which non-tungsten metals containedin the treatment object are dissolved may be employed; however, ifpossible, the potential is preferably controlled so that tungsten aloneis dissolved. That is, in the step of dissolving tungsten in a moltensalt, the potential of the anode and the cathode is preferablycontrolled to a predetermined value so that tungsten is selectivelydissolved in the molten salt. As a result, entry of impurities in thesubsequent deposition step can be reduced.

For this purpose, the molten salt is preferably selected such that, inthe step of dissolving, in the molten salt, tungsten from the treatmentobject, the difference between the standard electrode potential of asimple substance or alloy of tungsten and the standard electrodepotential of a simple substance or alloy of another metal in the moltensalt is 0.05 V or more. As a result, tungsten that is dissolved in themolten salt can be sufficiently separated from the metal element that isleft in the anode. The difference between the standard electrodepotentials is more preferably 0.1 V or more, still more preferably 0.25V or more.

The value of the potential controlled at the anode can be calculated byNernst equation described below.

The cathode used in the dissolution step is formed of carbon or amaterial that tends to form an alloy with an alkali metal such as Li orNa constituting cations in the molten salt. For example, aluminum (Al),zinc (Zn), gallium (Ga), cadmium (Cd), indium (In), tin (Sn), antimony(Sb), lead (Pb), or bismuth (Bi) may be used.

When the treatment object containing tungsten is used as an anode, forexample, the treatment object contained within a conductive basket(anode material) formed of metal or the like may be disposed in themolten salt. An opening may be formed in an upper portion of the basketso that the treatment object can be inserted through the opening intothe basket; and a large number of holes may be formed in the side andbottom walls of the basket so that the molten salt can flow into thebasket. The basket may be constituted by a desired material such as amesh member knitted from metal wires or a sheet member that is asheet-shaped metal plate having a large number of holes. In particular,it is effective that the material is formed of C, Pt, Mo, or the like.

In the cases where the object is an oxide or the like and has a highelectric resistance, the contact area between the object and theconductive material is preferably increased. The object is effectivelyused as an electrode by, for example, wrapping the object with a metalmesh member or filling the object into spaces within a metal porousmember.

The cathode and an anode formed of an anode material containing thetreatment object (for example, a metal basket containing the treatmentobject) are disposed in the molten salt; a control unit configured tocontrol the potential of the electrodes from the outside is connected;and the potential is controlled as described above. As a result,tungsten can be dissolved in the molten salt from the treatment object.

In the subsequent deposition process, molten salt electrolysis isperformed with a pair of electrode members disposed in the molten saltcontaining dissolved tungsten so that tungsten is deposited on one ofthe electrode members (cathode). In this case, by controlling thepotential value in the molten salt electrolysis, tungsten can beselectively deposited as metal or alloy on the cathode.

As in the dissolution process, in this deposition process, tungsten isseparated from other metals by utilizing the following characteristics:in molten salt electrolysis, different elements are deposited atdifferent potentials as metal or alloy on the cathode. Thus, even whenmetals other than tungsten are contained in the molten salt, bycontrolling the potential, tungsten alone can be deposited on thecathode. As a result, high purity tungsten can be obtained.

In deposition of tungsten, when the difference between thedissolution-deposition potential of tungsten and thedissolution-deposition potential of another metal contained in themolten salt is so small that tungsten is difficult to separate from themetal, the cathode material may be selected and the potential may becontrolled such that an alloy of the cathode material and tungsten isdeposited. As a result, tungsten in the molten salt can be separated asa tungsten alloy from the other impurity metal; and, after that, forexample, a dissolution step and a deposition step in another molten saltcan be performed with the cathode material alloyed with tungsten tothereby provide high purity tungsten.

The electrode members used in the deposition step may be formed of, forexample, nickel (Ni), molybdenum (Mo), or glassy carbon (C).

In this embodiment, the above-described two processes are used toseparate and extract tungsten from a treatment object. In thisembodiment, since a molten salt is used, the system needs to be heatedsuch that the temperature of the system in the processes is equal to ormore than the melting point of the molten salt.

Alternatively, smelting in the processes can be performed on the basisof a totally contrary idea. That is, a treatment object is used as theanode and only metal elements serving as impurities are dissolved in amolten salt. In this case, by also controlling the potential at theanode to a predetermined value, such a phenomenon is caused in whichtungsten is left in the anode and impurity elements are dissolved. As aresult, tungsten is provided in the anode.

A feature of the two processes is use of a molten salt. Thus, thecharacteristics of molten salt electrolysis in which different moltensalts have different dissolution-deposition potentials for elements areutilized; and the processes can be designed by selecting a molten saltsuch that the dissolution-deposition potential of tungsten and thedissolution-deposition potential of a non-tungsten impurity metal aresufficiently different values that allow easy performance of theprocesses.

Specifically, the molten salt is preferably selected such that, in thestep of depositing or alloying tungsten, the difference between thestandard electrode potential of a simple substance or alloy of tungstenand the standard electrode potential of a simple substance or alloy ofanother impurity metal in the molten salt is 0.05 V or more. Thedifference between the standard electrode potential of a simplesubstance or alloy of tungsten and the standard electrode potential of asimple substance or alloy of another metal in the molten salt is morepreferably 0.1 V or more, still more preferably 0.25 V or more.

In this way, in the step of depositing or alloying tungsten, thepotential of the electrode members is preferably controlled to apredetermined value so that the tungsten in the molten salt isselectively deposited or alloyed.

The deposition potential of tungsten to be deposited on the cathode canbe determined by electrochemical calculation. Specifically, thecalculation is performed with Nernst equation.

For example, the potential at which a simple substance of tungsten (W)is deposited from divalent W ions (hereafter represented by W(II)) canbe determined with the following equation.

E _(W) =E ⁰ _(W) +RT/3F·ln(a _(W(II)) /a _(W(0)))  Equation (1)

In Eq. (1), E⁰ _(W) represents the standard potential, R represents thegas constant, T represents absolute temperature, F represents theFaraday constant, a_(W(II)) represents the activity of W(II) ions, anda_(W(0)) represents the activity of W simple substance.

When Eq. (1) is rewritten in view of activity coefficient γ_(W(II)),since a_(W(0))=1, the following equations are provided.

$\begin{matrix}\begin{matrix}{E_{Wr} = {E_{W}^{0} + {{{RT}/3}\; {F \cdot \ln}\; a_{W{({II})}}}}} \\{= {E_{W}^{0} + {{{RT}/3}\; {F \cdot {\ln \left( {\gamma_{W{({II})}} \cdot C_{W{({II})}}} \right)}}}}}\end{matrix} & {{Equation}\mspace{14mu} (2)} \\{E_{W} = {E_{W}^{0^{\prime}} + {{{RT}/3}\; {F \cdot \ln}\; C_{W{({II})}}}}} & {{Equation}\mspace{14mu} (3)}\end{matrix}$

In Eq. (3), C_(W(II)) represents the concentration of divalent W ions,and E^(0′) _(W) represents formal electrode potential (here, equal to E⁰_(W)+RT/3F·ln γ_(W(II))).

Similarly, by using the above-described equations, deposition potentialsof all deposits corresponding to different molten salts can bedetermined. Similar calculations can also be performed in the case ofdepositing tungsten as an alloy. In the process of depositing oralloying tungsten on the cathode, in view of the deposition potentialvalues of tungsten simple substance and tungsten alloy, the molten saltand the cathode material are selected such that a sufficiently highpotential difference is achieved with respect to the depositionpotential of a simple substance or alloy of another metal, and whethertungsten is deposited or a tungsten alloy is deposited is decided.

Voltage and current during operation vary depending on the size orpositional relationship of electrodes. Accordingly, reference values ofvoltage and current are determined on the basis of conditions andsubsequently voltage and current are determined in each step on thebasis of the potential value and order determined by the above-describedmethod.

As described above, in a method for producing tungsten by molten saltelectrolysis according to this embodiment, the potential value iscontrolled to thereby electrochemically dissolve and deposit tungsten.Accordingly, the steps can be simplified, compared with, for example,the existing wet treatment involving repeating of processes ofdissolution and extraction using acid or the like; and the particularelement can be selectively separated and recovered. In addition,adjustment of the specific gravity of molten salt is not necessary; and,by selecting a low-temperature molten salt in which tungsten can betreated in the solid state, a simple apparatus configuration can beemployed. Moreover, the operation pattern can also be simplified. As aresult, the steps can be performed efficiently at low cost.

Alternatively, as described above, tungsten can be smelted on the basisof an idea that is totally contrary to the idea of depositing oralloying tungsten on the cathode.

That is, a method for producing a metal according to this embodiment isa method for producing tungsten by molten salt electrolysis from atreatment object containing tungsten, wherein a cathode and an anodethat is formed of an anode material containing the treatment object aredisposed in a molten salt, and the potential at the anode is controlledso that a metal element corresponding to the potential value isdissolved in the molten salt from the treatment object and tungsten isleft in the anode.

In this method, the anode material containing the treatment object isused as the anode and metal elements other than tungsten, that is, onlymetal elements serving as impurities are dissolved in the molten salt,so that tungsten is left in the anode. In this case, by also controllingthe potential at the anode, such a phenomenon can be caused in whichtungsten as the smelting target is left in the anode and impurityelements are dissolved in the molten salt. As a result, smelted tungstenis provided in the anode.

In this method, the molten salt is also preferably selected such that,in the step of dissolving, in the molten salt, a metal element from thetreatment object, the difference between the standard electrodepotential of a simple substance or alloy of tungsten and the standardelectrode potential of a simple substance or alloy of another metal inthe molten salt is 0.05 V or more. As a result, tungsten can besufficiently separated from the other metal and tungsten alone can beleft in the anode. The difference between the standard electrodepotentials is more preferably 0.1 V or more, still more preferably 0.25V or more.

The value of the potential controlled at the anode can be calculated byNernst equation as described above.

In a method for producing tungsten by molten salt electrolysis accordingto this embodiment, the treatment object containing tungsten ispreferably, for example, a metal material containing tungsten. Examplesof the metal material containing tungsten include tungsten heaters.

This embodiment is also suitably applicable to cases where the treatmentobject is a metal material containing tungsten and a transition metal.Such a transition metal is not particularly limited and may be anyelement among from group 3 (group IIIA) to group 11 (group IB) of theperiodic table. Examples of the metal material containing tungsten and atransition metal include cemented carbide.

The treatment object may be, for example, cemented carbide products.Herein, cemented carbide products collectively denote products includingcemented carbide materials, such as cutting tools, jigs, dies, and moldsincluding cemented carbide materials.

The molten salt can be selected from chloride molten salts and fluoridemolten salts. A molten salt mixture containing a chloride molten saltand a fluoride molten salt may be used.

Examples of chloride molten salts include KCl, NaCl, CaCl₂, LiCl, RbCl,CsCl, SrCl₂, BaCl₂, and MgCl₂. Examples of fluoride molten salts includeLiF, NaF, KF, RbF, CsF, MgF₂, CaF₂, SrF₂, and BaF₂. Chloride moltensalts are preferably used in view of efficiency; in particular, KCl,NaCl, and CaCl₂ are preferably used because they are inexpensive andeasily available.

Among such molten salts, a plurality of molten salts can be combined andused as a molten salt having a desirable composition. For example, amolten salt having a composition such as KCl—CaCl₂, LiCl—KCl, orNaCl—KCl may be used.

In a method for producing tungsten by molten salt electrolysis accordingto this embodiment, the following apparatuses can be preferably used.That is, an apparatus used for a method for producing tungsten by moltensalt electrolysis according to this embodiment includes a containercontaining a molten salt; a cathode immersed in the molten saltcontained within the container; and an anode that is immersed in themolten salt contained within the container and that contains aconductive treatment object containing tungsten, wherein the molten saltis movable into and out of the anode, the apparatus further includes acontrol unit configured to control the potential of the cathode and theanode to a predetermined value, and the value of the potential ischangeable in the control unit. An apparatus used for a method forproducing tungsten by molten salt electrolysis according to thisembodiment includes a container containing a molten salt containingdissolved tungsten; and a cathode and an anode that are immersed in themolten salt contained within the container, wherein the apparatusincludes a control unit configured to control the potential of thecathode and the anode to a predetermined value, and the value of thepotential is changeable in the control unit.

The apparatuses for this embodiment will be described with reference toFIGS. 18 and 19. An apparatus illustrated in FIG. 18 includes acontainer 1 containing a molten salt, a molten salt 2 contained withinthe container 1, a basket 4 containing a treatment object 3 containingtungsten, an electrode 6, a heater 10 for heating the molten salt 2, anda control unit 9 electrically connected to the basket 4 and theelectrode 6 via conductive wires 5.

The control unit 9 is configured to control the potential of oneelectrode (anode) that is the basket 4 and the other electrode (cathode)that is the electrode 6, to a predetermined value. In the control unit9, the value to which the potential is controlled is changeable. Theheater 10 is disposed so as to circularly surround the container 1. Theelectrode 6 may be formed of a desired material, for example, carbon.The container 1 may have a bottom surface that has a circular shape or apolygonal shape. The basket 4 may be the above-described basket.

The potential of the basket 4 and the electrode 6 is controlled by thecontrol unit 9 to a predetermined potential value. As a result, tungstenis dissolved in the molten salt 2 from the treatment object 3.

After tungsten is sufficiently dissolved from the treatment object 3,the basket 4 and the electrode 6 are removed and another electrode 7(cathode) and another electrode 8 (anode) are placed in the molten salt2. These electrodes 7 and 8 are connected to the control unit 9 viaconductive wires 5. The control unit 9 is used to control the potentialof the electrodes 7 and 8 to a predetermined value. At this time, thepotential is controlled such that the potential at the electrode 7 isthe deposition potential of tungsten. As a result, tungsten dissolved inthe molten salt 2 is deposited on the surface of the electrode 7(cathode). The electrodes 7 and 8 may be formed of a material such asglassy carbon (C).

The heating temperature for the molten salt 2 with the heater 10 may be,for example, 800° C. in both of the treatments using the apparatusesillustrated in FIGS. 18 and 19. In this way, tungsten can be depositedas a simple substance on the surface of the electrode 7.

The potential of the electrodes 7 and 8 may be controlled such that analloy of tungsten and the cathode material is deposited on the surfaceof the electrode 7 (cathode). In this case, the above-describeddissolution step and deposition step may be performed with the alloyedelectrode 7. That is, the apparatus illustrated in FIG. 18 is newlyprepared and the electrode 7 alloyed with tungsten is used instead ofthe above-described treatment object 3.

In the case where the method for producing tungsten of this embodimentis performed with the apparatuses illustrated in FIGS. 18 and 19, forexample, the method may be performed in the following manner.

Cemented carbide cutting tools (9 kg) are first prepared as thetreatment object 3 and KCl—NaCl is prepared as the molten salt 2. Forexample, the cemented carbide cutting tools may contain 90 wt % oftungsten carbide (WC) and 10 wt % of cobalt (Co). The cemented carbidecutting tools are ground and placed within the basket 4. From theviewpoint of enhancement of the treatment efficiency, the size of thecemented carbide cutting tools serving as the treatment object 3 ispreferably minimized by grinding. For example, the cemented carbidecutting tools are ground into particles having a maximum particle sizeof 5 mm or less, preferably 3 mm or less, more preferably 1 mm or less.The amount of the molten salt 2 is about 16 liters (mass: 25 kg).

The above-described dissolution step may be performed with a carbonelectrode serving as the electrode 6. Subsequently, the deposition stepmay be performed with electrodes formed of glassy carbon and serving asthe electrodes 7 and 8.

As has been described, tungsten can be recovered from cemented carbidecutting tools serving as the treatment object 3. According to the methodfor producing tungsten by molten salt electrolysis of this embodiment,the apparatus configuration can be simplified and the treatment time canalso be decreased, compared with the existing wet separation method andthe like. Thus, the cost incurred can be reduced. In addition, byappropriately setting a potential at an electrode, tungsten can bedeposited as a simple substance on the electrode surface and hence highpurity tungsten can be obtained. The potentials for depositing tungstenand a tungsten alloy can be determined by the above-describedcalculation.

Third Embodiment

A method for producing lithium by molten salt electrolysis according tothis embodiment is a method for producing lithium by molten saltelectrolysis from a treatment object containing lithium, the methodincluding a step of dissolving, in a molten salt, lithium from thetreatment object, and a step of depositing lithium present in the moltensalt, on one of a pair of electrode members disposed in the molten saltcontaining dissolved lithium, by controlling a potential of theelectrode members to a predetermined value.

That is, the lithium production method of this embodiment includes aprocess of dissolving, in a molten salt, lithium contained in thetreatment object, and a step of depositing lithium on one of electrodes(cathode) from the molten salt containing dissolved lithium by moltensalt electrolysis. A feature of this embodiment is that, by controllingthe potential of the electrodes in the step of dissolving lithium,lithium is selectively dissolved from the treatment object; and, bycontrolling the potential of the electrodes to a predetermined value inthe step of depositing lithium, lithium is selectively deposited on thecathode from the molten salt to thereby provide high purity lithium.

The step of dissolving, in a molten salt, lithium contained in atreatment object will be first described.

A procedure for dissolving, in a molten salt, lithium contained in atreatment object is, for example, a chemical procedure for dissolution.Specifically, a treatment object is ground into particles or powder,mixed with a salt, and heated. As a result, lithium contained in thetreatment object can be dissolved in the molten salt. Alternatively, atreatment object may be placed in a molten salt and dissolved.

Another procedure is an electrochemical procedure. Specifically, ananode formed of an anode material containing a treatment object isplaced in a molten salt and the value of the potential at the treatmentobject placed as the anode is controlled to selectively dissolve lithiumcontained in the treatment object. Molten salt electrolysis ischaracterized in that different elements are dissolved at differentpotentials. Accordingly, in this way, by using a treatment object as ananode and controlling the potential during dissolution, lithium can beselectively dissolved in a molten salt to separate lithium from theother metals.

In this step, the entire treatment object may be dissolved, or alithium-containing portion of the treatment object or lithium alone maybe dissolved. Non-lithium metals contained in the treatment object mayalso be dissolved; however, if possible, the potential value ispreferably controlled so that lithium alone is dissolved. That is, inthe step of dissolving lithium in a molten salt, the potential of theanode and the cathode is preferably controlled to a predetermined valueso that lithium is selectively dissolved in the molten salt. As aresult, entry of impurities in the subsequent deposition step can bereduced.

For this purpose, the molten salt is preferably selected such that, inthe step of dissolving, in the molten salt, lithium from the treatmentobject, the difference between the standard electrode potential of asimple substance or alloy of lithium and the standard electrodepotential of a simple substance or alloy of another metal in the moltensalt is 0.05 V or more. As a result, lithium that is dissolved in themolten salt can be sufficiently separated from the metal element that isleft in the anode. The difference between the standard electrodepotentials is more preferably 0.1 V or more, still more preferably 0.25V or more.

The value of the potential controlled at the anode can be calculated byNernst equation described below.

The cathode used in the dissolution step is formed of carbon or amaterial that tends to form an alloy with an alkali metal such as Li orNa constituting cations in the molten salt. For example, aluminum (Al),zinc (Zn), gallium (Ga), cadmium (Cd), indium (In), tin (Sn), antimony(Sb), lead (Pb), or bismuth (Bi) may be used.

When the treatment object containing lithium is used as an anode, forexample, the treatment object contained within a conductive basket(anode material) formed of metal or the like may be disposed in themolten salt. An opening may be formed in an upper portion of the basketso that the treatment object can be inserted through the opening intothe basket; and a large number of holes may be formed in the side andbottom walls of the basket so that the molten salt can flow into thebasket. The basket may be constituted by a desired material such as amesh member knitted from metal wires or a sheet member that is asheet-shaped metal plate having a large number of holes. In particular,it is effective that the material is formed of C, Pt, Mo, or the like.

In the cases where the object is an oxide or the like and has a highelectric resistance, the contact area between the object and theconductive material is preferably increased. The object is effectivelyused as an electrode by, for example, wrapping the object with a metalmesh member or filling the object into spaces within a metal porousmember.

The cathode and an anode formed of an anode material containing thetreatment object (for example, a metal basket containing the treatmentobject) are disposed in the molten salt; a control unit configured tocontrol the potential of the electrodes from the outside to apredetermined value is connected; and the potential is controlled asdescribed above. As a result, lithium can be dissolved in the moltensalt from the treatment object.

In the subsequent deposition step, molten salt electrolysis is performedwith a pair of electrode members disposed in the molten salt containingdissolved lithium so that lithium is deposited on one of the electrodemembers (cathode). In this case, by controlling the potential value inthe molten salt electrolysis, lithium can be selectively deposited asmetal or alloy on the cathode.

As in the dissolution step, in this deposition step, lithium isseparated from other metals by utilizing the following characteristics:in molten salt electrolysis, different elements are deposited atdifferent potentials as metal or alloy on the cathode. Thus, even whenmetals other than lithium are contained in the molten salt, bycontrolling the potential, lithium alone can be deposited on thecathode. As a result, high purity lithium can be obtained.

In deposition of lithium, when the difference between thedissolution-deposition potential of lithium and thedissolution-deposition potential of another metal contained in themolten salt is so small that lithium is difficult to separate from themetal, the cathode material may be selected and the potential may becontrolled such that an alloy of the cathode material and lithium isdeposited. As a result, lithium in the molten salt can be separated as alithium alloy from the other impurity metal; and, after that, adissolution step and a deposition step in another molten salt areperformed with the cathode material alloyed with lithium to therebyprovide high purity lithium.

The electrode members used in the deposition step may be formed of, forexample, nickel (Ni), molybdenum (Mo), or glassy carbon (C).

In this embodiment, the above-described two steps are used to separateand recover lithium from a treatment object.

In this embodiment, since a molten salt is used, the system needs to beheated such that the temperature of the system in the steps is equal toor more than the melting point of the molten salt.

A feature of the two steps is use of a molten salt as the electrolyticsolution. Thus, the characteristics of molten salt electrolysis in whichdifferent molten salts have different dissolution-deposition potentialsfor elements are utilized; and the steps can be designed by selecting amolten salt such that the dissolution-deposition potential of lithiumand the dissolution-deposition potential of a non-lithium impurity metalare sufficiently different values that allow easy performance of thesteps.

Specifically, the molten salt is preferably selected such that, in thestep of depositing or alloying lithium, the difference between thestandard electrode potential of a simple substance or alloy of lithiumand the standard electrode potential of a simple substance or alloy ofanother impurity metal in the molten salt is 0.05 V or more. Thedifference between the standard electrode potential of a simplesubstance or alloy of lithium and the standard electrode potential of asimple substance or alloy of another metal in the molten salt is morepreferably 0.1 V or more, still more preferably 0.25 V or more.

In this way, in the step of depositing or alloying lithium, thepotential of the electrode members is preferably controlled to apredetermined value so that the lithium in the molten salt isselectively deposited or alloyed.

The deposition potential of lithium to be deposited on the cathode canbe determined by electrochemical calculation. Specifically, thecalculation is performed with Nernst equation.

For example, the potential at which a simple substance of Li isdeposited from lithium ions (Li⁺) can be determined with the followingequation.

E _(Li) =E ⁰ _(Li) +RT/3F·ln(a _(Li(I)) /a _(Li(0)))  Equation (1)

In Eq. (1), E⁰ _(Li) represents the standard potential, R represents thegas constant, T represents absolute temperature, F represents theFaraday constant, a_(Li(I)) represents the activity of Li ions, anda_(Li(0)) represents the activity of Li simple substance.

When Eq. (1) is rewritten in view of activity coefficient γ_(Li(I)),since a_(Li(0))=1, the following equations are provided.

$\begin{matrix}\begin{matrix}{E_{Li} = {E_{Li}^{0} + {{{RT}/3}\; {F \cdot \ln}\; a_{{Li}{(I)}}}}} \\{= {E_{Li}^{0} + {{{RT}/3}\; {F \cdot {\ln \left( {\gamma_{{Li}{(I)}} \cdot C_{{Li}{(I)}}} \right)}}}}}\end{matrix} & {{Equation}\mspace{14mu} (2)} \\{E_{Li} = {E_{Li}^{0^{\prime}} + {{{RT}/3}\; {F \cdot \ln}\; C_{{Li}{(I)}}}}} & {{Equation}\mspace{14mu} (3)}\end{matrix}$

In Eq. (3), C_(Li(I)) represents the concentration of Li ions, andE^(0′) _(Li) represents formal electrode potential (here, equal to E⁰_(Li)+RT/3F·ln γ_(Li(I))).

Similarly, in the case where LiM alloy (M represents an alloyed metal)is deposited on the electrode surface, the potential (depositionpotential: E_(LiM)) can be determined with the following equation.

E _(Li.M) =E ^(0′) _(Li.M) +RT/3F·ln C _(Li(I))  Equation (4)

In Eq. (4), E^(0′) _(Li.M) represents formal electrode potential (here,equal to E⁰ _(Li.M)+RT/3F·ln γ_(Li(I))).

Similarly, by using the above-described equations, deposition potentialsof all deposits corresponding to different molten salts can bedetermined. In the step of depositing or alloying lithium on thecathode, in view of the deposition potential values of lithium simplesubstance and lithium alloy, the molten salt and the cathode materialare selected such that a sufficiently high potential difference isachieved with respect to the deposition potential of a simple substanceor alloy of another metal, and whether lithium is deposited or a lithiumalloy is deposited is decided.

Voltage and current during operation vary depending on the size orpositional relationship of electrodes. Accordingly, reference values ofvoltage and current are determined on the basis of conditions andsubsequently voltage and current are determined in each step on thebasis of the potential value and order determined by the above-describedmethod.

As described above, in a method for producing lithium by molten saltelectrolysis according to this embodiment, the potential value iscontrolled to thereby electrochemically dissolve and deposit lithium.Accordingly, the steps can be simplified, compared with, for example,the existing wet treatment involving repeating of steps of dissolutionand extraction using acid or the like; and the particular element can beselectively separated and recovered. In addition, adjustment of thespecific gravity of molten salt is not necessary; and, by selecting alow-temperature molten salt in which lithium can be treated in the solidstate, a simple apparatus configuration can be employed. Moreover, theoperation pattern can also be simplified. As a result, the steps can beperformed efficiently at low cost.

In a method for producing lithium by molten salt electrolysis accordingto this embodiment, the treatment object is not limited as long as it isa material containing lithium. Preferred examples of the treatmentobject include negative electrode materials of lithium primary batteriesand positive electrode materials of lithium-ion secondary batteries.

Examples of positive electrode active materials of positive electrodematerials of thium-ion secondary batteries include lithium cobalt oxide(LiCoO₂), lithium nickel oxide (LiNiO₂), lithium nickel cobalt oxide(LiCo_(0.3)Ni_(0.7)O₂), lithium manganese oxide (LiMn₂O₄), lithiumtitanium oxide (Li₄Ti₅O₁₂), lithium manganese oxide compounds(LiM_(y)Mn_(2-y)O₄); M=Cr, Co, Ni), and lithium acid.

The molten salt can be selected from chloride molten salts and fluoridemolten salts. A molten salt mixture containing a chloride molten saltand a fluoride molten salt may be used.

Examples of chloride molten salts include KCl, NaCl, CaCl₂, LiCl, RbCl,CsCl, SrCl₂, BaCl₂, and MgCl₂. Examples of fluoride molten salts includeLiF, NaF, KF, RbF, CsF, MgF₂, CaF₂, SrF₂, and BaF₂. Chloride moltensalts are preferably used in view of efficiency; in particular, KCl,NaCl, and CaCl₂ are preferably used because they are inexpensive andeasily available.

Among such molten salts, a plurality of molten salts can be combined andused as a molten salt having a desirable composition. For example, amolten salt having a composition such as KCl—CaCl₂, LiCl—KCl, orNaCl—KCl may be used.

In a method for producing lithium by molten salt electrolysis accordingto this embodiment, the following apparatuses can be preferably used.That is, an apparatus used for a method for producing lithium by moltensalt electrolysis according to this embodiment includes a containercontaining a molten salt; a cathode immersed in the molten saltcontained within the container; and an anode that is immersed in themolten salt contained within the container and that contains aconductive treatment object containing lithium, wherein the molten saltis movable into and out of the anode, the apparatus further includes acontrol unit configured to control the potential of the cathode and theanode to a predetermined value, and the value of the potential ischangeable in the control unit.

An apparatus used for a method for producing lithium by molten saltelectrolysis according to this embodiment includes a containercontaining a molten salt containing dissolved lithium; and a cathode andan anode that are immersed in the molten salt contained within thecontainer, wherein the apparatus includes a control unit configured tocontrol the potential of the cathode and the anode to a predeterminedvalue, and the value of the potential is changeable in the control unit.

The apparatuses for this embodiment will be described with reference toFIGS. 18 and 19. An apparatus illustrated in FIG. 18 includes acontainer 1 containing a molten salt, a molten salt 2 contained withinthe container 1, a basket 4 containing a treatment object 3 containinglithium, an electrode 6, a heater 10 for heating the molten salt 2, anda control unit 9 electrically connected to the basket 4 and theelectrode 6 via conductive wires 5.

The control unit 9 is configured to control the potential of oneelectrode (anode) that is the basket 4 and the other electrode (cathode)that is the electrode 6, to a predetermined value. In the control unit9, the value to which the potential is controlled is changeable. Theheater 10 is disposed so as to circularly surround the container 1. Theelectrode 6 may be formed of a desired material, for example, aluminum.The container 1 may have a bottom surface that has a circular shape or apolygonal shape. The basket 4 may be the above-described basket.

The potential of the basket 4 and the electrode 6 is controlled by thecontrol unit 9 to a predetermined potential value. As a result, lithiumis dissolved in the molten salt 2 from the treatment object 3.

After lithium is sufficiently dissolved from the treatment object 3, thebasket 4 and the electrode 6 are removed and, as illustrated in FIG. 19,another electrode 7 (cathode) and another electrode 8 (anode) are placedin the molten salt 2. These electrodes 7 and 8 are connected to thecontrol unit 9 via conductive wires 5. The control unit 9 is used tocontrol the potential of the electrodes 7 and 8 to a predeterminedvalue. At this time, the potential is controlled such that the potentialat the electrode 7 is the deposition potential of lithium. As a result,lithium dissolved in the molten salt 2 is deposited on the surface ofthe electrode 7 (cathode). The electrodes 7 and 8 may be formed of amaterial such as glassy carbon (C).

The heating temperature for the molten salt 2 with the heater 10 may be,for example, 800° C. in both of the treatments using the apparatusesillustrated in FIGS. 18 and 19. In this way, lithium can be deposited asa simple substance on the surface of the electrode 7.

The potential of the electrodes 7 and 8 may be controlled to a valuesuch that an alloy of lithium and the cathode material is deposited onthe surface of the electrode 7 (cathode). In this case, theabove-described dissolution step and deposition step may be performedwith the alloyed electrode 7. That is, the apparatus illustrated in FIG.18 is newly prepared and the electrode 7 alloyed with lithium is usedinstead of the above-described treatment object 3.

In the case where the method for producing lithium of this embodiment isperformed with the apparatuses illustrated in FIGS. 18 and 19, forexample, the method may be performed in the following manner.

A lithium-containing positive electrode material of lithium-ionbatteries is first prepared as the treatment object 3 and KCl—NaCl isprepared as the molten salt 2. For example, the positive electrodematerial is a powder containing lithium cobalt oxide (LiCoO₂) or lithiummanganese oxide. The positive electrode material is ground and placedwithin the basket 4. From the viewpoint of enhancement of the treatmentefficiency, the size of the positive electrode material serving as thetreatment object 3 is preferably minimized by grinding. For example, thepositive electrode material is ground into particles having a maximumparticle size of 5 mm or less, preferably 3 mm or less, more preferably1 mm or less. The above-described dissolution step may be performed witha carbon electrode serving as the electrode 6. Subsequently, thedeposition step may be performed with electrodes formed of glassy carbonand serving as the electrodes 7 and 8.

As has been described, lithium can be recovered from the positiveelectrode material serving as the treatment object 3.

According to the method for producing lithium by molten saltelectrolysis of this embodiment, the apparatus configuration can besimplified and the treatment time can also be decreased, compared withthe existing wet separation method and the like. Thus, the cost incurredcan be reduced. In addition, by appropriately setting a potential valueat an electrode, lithium can be deposited as a simple substance on theelectrode surface and hence high purity lithium can be obtained.

Fourth Embodiment

This embodiment is a method for producing a metal by molten saltelectrolysis, the method including a step of dissolving, in a moltensalt, a metal element contained in a treatment object containing two ormore metal elements; and a step of depositing or alloying a particularmetal present in the molten salt, on one of a pair of electrode membersdisposed in the molten salt containing the dissolved metal element, bycontrolling a potential of the electrode members to a predeterminedvalue.

Roughly speaking, this embodiment includes a process of dissolving, in amolten salt, a particular metal contained in the treatment object, and aprocess of depositing the particular metal on one of electrodes(cathode) from the molten salt containing the dissolved particular metalby molten salt electrolysis. A feature of this embodiment is that, bycontrolling the potential of the electrodes to a predetermined value,the particular metal is selectively deposited from the treatment objectto provide the particular metal at high purity.

The process of dissolving, in a molten salt, a particular metalcontained in a treatment object will be first described.

A procedure for dissolving, in a molten salt, a particular metalcontained in a treatment object is, for example, a chemical procedurefor dissolution. Specifically, a treatment object is ground intoparticles or powder, mixed with a salt, and heated. As a result, theparticular metal contained in the treatment object can be dissolved inthe molten salt. Alternatively, the treatment object may be placed in amolten salt and dissolved.

Another procedure is an electrochemical procedure. Specifically, acathode and an anode that is formed of an anode material containing thetreatment object are disposed in the molten salt; and the potential atthe anode is controlled to a predetermined value so that a metal elementcorresponding to the controlled potential value is dissolved in themolten salt from the treatment object. Molten salt electrolysis ischaracterized in that different elements are dissolved at differentpotentials; and such characteristics are utilized to thereby separate aparticular metal from other metals. In this way, by using a treatmentobject as an anode and controlling the potential during dissolution, aparticular metal can be selectively dissolved in a molten salt.

In this step, all the metals contained in the treatment object may bedissolved. Alternatively, a particular metal and another metal containedin the treatment object may be dissolved. Preferably, only a particularmetal contained in the treatment object is dissolved. Conditions underwhich a particular metal and another metal contained in the treatmentobject are dissolved may be employed; however, if possible, thepotential is preferably controlled so that the particular metal alone isdissolved. That is, in the step of dissolving a particular metal in amolten salt, the potential at the anode is preferably controlled to apredetermined value so that the particular metal element is selectivelydissolved in the molten salt. As a result, entry of impurities in thesubsequent deposition step can be reduced.

For this purpose, the molten salt is preferably selected such that, inthe step of dissolving, in the molten salt, a particular metal from thetreatment object, the difference between the standard electrodepotential of a simple substance or alloy of the particular metal and thestandard electrode potential of a simple substance or alloy of anothermetal in the molten salt is 0.05 V or more. As a result, the particularmetal that is dissolved in the molten salt can be sufficiently separatedfrom the other metal element that is left in the anode. The differencebetween the standard electrode potentials is more preferably 0.1 V ormore, still more preferably 0.25 V or more.

The value of the potential controlled at the anode can be calculated byNernst equation described below.

When one or more target particular metals are contained in the treatmentobject, in the dissolution step, one or more particular metals aredissolved in the molten salt.

When the treatment object contains only one particular metal, asdescribed above, this particular metal is dissolved and then thedeposition step is performed to provide the target metal. When thetreatment object contains two or more target particular metals, only oneof the metals may be dissolved in a molten salt; a deposition step maybe subsequently performed; and, after that, another dissolution step maybe performed so that the remainder of the particular metals is dissolvedin the molten salt. In this case, the treatment object having been usedin the initial dissolution step may be moved from the molten salt usedin this dissolution step to another molten salt and subjected to adissolution step to thereby dissolve the remainder of the particularmetals.

When two or more particular metals contained in the treatment object aredissolved in a molten salt, the subsequent deposition step may beperformed such that the particular metals present in the molten salt aredeposited or alloyed one by one on electrode materials, so that desiredparticular metals can be produced. In this case, after one particularmetal is deposited or alloyed on an electrode material, this electrodematerial may be replaced by another electrode material and anotherparticular metal dissolved in the molten salt may be deposited oralloyed on this electrode material.

The cathode used in the dissolution step is formed of carbon or amaterial that tends to form an alloy with an alkali metal such as Li orNa constituting cations in the molten salt. For example, aluminum (Al),zinc (Zn), gallium (Ga), cadmium (Cd), indium (In), tin (Sn), antimony(Sb), lead (Pb), or bismuth (Bi) may be used.

When the treatment object containing a particular metal is used as ananode, for example, the treatment object contained within a conductivebasket (anode material) formed of metal or the like may be disposed inthe molten salt. An opening may be formed in an upper portion of thebasket so that the treatment object can be inserted through the openinginto the basket; and a large number of holes may be formed in the sideand bottom walls of the basket so that the molten salt can flow into thebasket. The basket may be constituted by a desired material such as amesh member knitted from metal wires or a sheet member that is asheet-shaped metal plate having a large number of holes. In particular,it is effective that the material is formed of C, Pt, Mo, or the like.

In the cases where the object is an oxide or the like and has a highelectric resistance, the contact area between the object and theconductive material is preferably increased. The object is effectivelyused as an electrode by, for example, wrapping the object with a metalmesh member or filling the object into spaces within a metal porousmember.

The cathode and an anode formed of an anode material containing thetreatment object (for example, a metal basket containing the treatmentobject) are disposed in the molten salt; and the potential at the anodeis controlled to a predetermined value. As a result, a particular metalcan be dissolved in the molten salt from the treatment object.

In the subsequent deposition process, molten salt electrolysis isperformed with a pair of electrode members disposed in the molten saltcontaining the dissolved particular metal so that the particular metalis deposited on one of the electrode members (cathode). In this case, bycontrolling the potential value in the molten salt electrolysis, theparticular metal can be selectively deposited as metal or alloy on thecathode.

As in the dissolution process, in this deposition process, theparticular metal is separated from other metals by utilizing thefollowing characteristics: in molten salt electrolysis, differentelements are deposited at different potentials as metal or alloy on thecathode. Thus, even when metals other than the particular metal arecontained in the molten salt, by controlling the potential of theelectrode members to a predetermined value, the particular metal elementcan be selectively deposited or alloyed on the cathode. That is, theparticular metal at high purity can be obtained.

In deposition of a particular metal, when the difference between thedissolution-deposition potential of the particular metal and thedissolution-deposition potential of another metal contained in themolten salt is so small that the particular metal is difficult toseparate from the other metal, the cathode material may be selected andthe potential may be controlled such that an alloy of the cathodematerial and the particular metal is deposited. As a result, theparticular metal in the molten salt can be deposited as an alloy andseparated from the other impurity metal; and, after that, for example, adissolution step and a deposition step in another molten salt can beperformed with the cathode material alloyed with the particular metal tothereby provide the particular metal at high purity.

The electrode members used in the deposition step may be formed of, forexample, nickel (Ni), molybdenum (Mo), or glassy carbon (C).

In this embodiment, the above-described two processes are used toseparate and extract a particular metal from a treatment object. In thisembodiment, since a molten salt is used, the system needs to be heatedsuch that the temperature of the system in the processes is equal to ormore than the melting point of the molten salt.

Alternatively, as described below, smelting can be performed on thebasis of an idea that is totally contrary to that of the processes. Thatis, a treatment object is used as the anode and only metal elementsserving as impurities are dissolved in a molten salt. In this case, byalso controlling the potential at the anode, such a phenomenon is causedin which a particular metal is left in the anode and impurity elementsare dissolved. As a result, the particular metal is provided in theanode.

A feature of the two processes is use of a molten salt. Thus, thecharacteristics of molten salt electrolysis in which different moltensalts have different dissolution-deposition potentials for elements areutilized; and the processes can be designed by selecting a molten saltsuch that the dissolution-deposition potential of a particular metal andthe dissolution-deposition potential of an impurity metal other than theparticular metal are sufficiently different values that allow easyperformance of the processes.

Specifically, the molten salt is preferably selected such that, in thestep of depositing or alloying a particular metal, the differencebetween the standard electrode potential of a simple substance or alloyof the particular metal and the standard electrode potential of a simplesubstance or alloy of another metal in the molten salt is 0.05 V ormore.

The difference between the standard electrode potential of a simplesubstance or alloy of the particular metal and the standard electrodepotential of a simple substance or alloy of another metal in the moltensalt is more preferably 0.1 V or more, still more preferably 0.25 V ormore.

In this way, in the step of depositing or alloying a particular metal,the potential of the electrode members is preferably controlled to apredetermined value so that the particular metal element in the moltensalt is selectively deposited or alloyed.

The deposition potential of a particular metal to be deposited on thecathode can be determined by electrochemical calculation. Specifically,the calculation is performed with Nernst equation.

For example, the potential at which a simple substance of molybdenum(Mo) serving as the particular metal is deposited from a molten salt inwhich molybdenum is dissolved into tetravalent Mo ions (hereafterrepresented by Mo(IV)) can be determined with the following equation.

E _(Mo) =E ⁰ _(Mo) +RT/3F·ln(a _(Mo(IV)) /a _(Mo(0)))  Equation (1)

In Eq. (1), E⁰ _(Mo) represents the standard potential, R represents thegas constant, T represents absolute temperature, F represents theFaraday constant, a_(Mo(IV)) represents the activity of Mo(IV) ions, anda_(Mo(0)) represents the activity of Mo simple substance.

When Eq. (1) is rewritten in view of activity coefficient γ_(Mo(Iv)),since a_(Mo(0))=1, the following equations are provided.

$\begin{matrix}\begin{matrix}{E_{Mo} = {E_{Mo}^{0} + {{{RT}/3}\; {F \cdot \ln}\; a_{{Mo}{({IV})}}}}} \\{= {E_{Mo}^{0} + {{{RT}/3}\; {F \cdot {\ln \left( {\gamma_{{Mo}{({IV})}} \cdot C_{{Mo}{({IV})}}} \right)}}}}}\end{matrix} & {{Equation}\mspace{14mu} (2)} \\{E_{Mo} = {E_{Mo}^{0^{\prime}} + {{{RT}/3}\; {F \cdot \ln}\; C_{{Mo}{({IV})}}}}} & {{Equation}\mspace{14mu} (3)}\end{matrix}$

In Eq. (3), C_(Mo(IV)) represents the concentration of tetravalent Moions, and E^(0′) _(Mo) represents formal electrode potential (here,equal to E⁰ _(Mo)+RT/3F·ln γ_(Mo(IV))).

Similarly, by using the above-described equations, deposition potentialsof all deposits corresponding to different molten salts can bedetermined.

Similar calculations can also be performed in the case of depositingmolybdenum as an alloy.

In the process of depositing or alloying molybdenum on the cathode, inview of the deposition potential values of molybdenum simple substanceand molybdenum alloy, the molten salt and the cathode material areselected such that a sufficiently high potential difference is achievedwith respect to the deposition potential of a simple substance or alloyof another metal, and whether molybdenum simple substance is depositedor a molybdenum alloy is deposited is decided.

Voltage and current during operation vary depending on the size orpositional relationship of electrodes. Accordingly, reference values ofvoltage and current are determined on the basis of conditions andsubsequently voltage and current are determined in each step on thebasis of the potential value and order determined by the above-describedmethod.

As described above, in a method for producing a particular metal bymolten salt electrolysis according to this embodiment, the potentialvalue is controlled to thereby electrochemically dissolve and depositthe particular metal. Accordingly, the steps can be simplified, comparedwith, for example, the existing wet treatment involving repeating ofprocesses of dissolution and extraction using acid or the like; and aparticular metal can be selectively separated and recovered. Inaddition, adjustment of the specific gravity of molten salt is notnecessary; and, by selecting a low-temperature molten salt in which theparticular metal can be treated in the solid state, a simple apparatusconfiguration can be employed. Moreover, the operation pattern can alsobe simplified. As a result, the steps can be performed efficiently atlow cost.

Alternatively, as described above, a particular metal can be smelted onthe basis of an idea that is totally contrary to the idea of depositingor alloying a particular metal on the cathode.

That is, a method for producing a metal by molten salt electrolysisaccording to this embodiment is a method for producing a particularmetal by molten salt electrolysis from a treatment object containing twoor more metal elements, wherein a cathode and an anode that is formed ofan anode material containing the treatment object are disposed in amolten salt, and the potential at the anode is controlled to apredetermined value so that a metal element corresponding to thepotential is dissolved in the molten salt from the treatment object andthe particular metal is left in the anode.

In this production method, the anode material containing the treatmentobject is used as the anode and metal elements other than the particularmetal, that is, only metal elements serving as impurities are dissolvedin the molten salt, so that the particular metal is left in the anode.In this case, by also controlling the potential at the anode, such aphenomenon can be caused in which the particular metal as the smeltingtarget is left in the anode and impurity elements are dissolved in themolten salt. As a result, the smelted particular metal is provided inthe anode.

In this method, the molten salt is also preferably selected such that,in the step of dissolving, in the molten salt, a metal element from thetreatment object, the difference between the standard electrodepotential of a simple substance or alloy of the particular metal and thestandard electrode potential of a simple substance or alloy of anothermetal in the molten salt is 0.05 V or more. As a result, the particularmetal can be sufficiently separated from the other metal and theparticular metal alone can be left in the anode. The difference betweenthe standard electrode potentials is more preferably 0.1 V or more,still more preferably 0.25 V or more.

The value of the potential controlled at the anode can be calculated byNernst equation as described above.

In a method for producing a metal by molten salt electrolysis accordingto this embodiment, the treatment object containing two or more metalelements is not limited at all as long as it is a metal materialcontaining a target particular metal. For example, Mn, Co, Sb, and thelike can be obtained from collected battery materials; Nb and the likecan be obtained from metal superconducting materials; Bi, Sr, and thelike can be obtained from oxide superconducting materials; V can beobtained from ferrovanadium; Mo and the like can be obtained from Mo—Cuheat spreaders; and Ge and the like can be obtained from optical fibermaterials.

This embodiment is also suitably applicable to cases where the treatmentobject is a metal material containing a transition metal or a rare earthmetal. Such a transition metal is not particularly limited and may beany element among from group 3 (group IIIA) to group 11 (group IB) ofthe periodic table. This embodiment is also suitably applicable to caseswhere the treatment object contains, as a transition metal, one or moremetals selected from the group consisting of V, Nb, Mo, Ti, Ta, Zr, andHf.

In addition, this embodiment is also suitably applicable to cases wherethe treatment object contains a metal that is one or both of Sr and Ba.Furthermore, this embodiment is also suitably applicable to cases wherethe treatment object contains one or more metals selected from the groupconsisting of Zn, Cd, Ga, In, Ge, Sn, Pb, Sb, and Bi.

In a method for producing a metal by molten salt electrolysis of thisembodiment, by selecting a transition metal or a rare earth metal as theparticular metal to be deposited or alloyed, the transition metal or therare earth metal can be obtained. Such a transition metal is notparticularly limited and may be any element among from group 3 (groupIIIA) to group 11 (group IB) of the periodic table.

Similarly, by selecting the particular metal to be deposited or alloyedfrom V, Nb, Mo, Ti, Ta, Zr and Hf, or Sr and Ba, or Zn, Cd, Ga, In, Ge,Sn, Pb, Sb, and Bi, such metals can be obtained.

As described above, in the dissolution step, one or more of these metalscontained in the treatment object can be dissolved in a molten salt andparticular metals can be sequentially deposited or alloyed on electrodemembers from the molten salt.

The treatment object preferably has the form of particles or powder.When the treatment object is prepared so as to have the form ofparticles or powder, the surface area is increased and the treatmentefficiency can be increased.

In addition, the treatment object prepared in the form of particles orpowder can be compacted and used as the anode. In this case, between theparticles, there are desirably spaces that the molten salt can easilyenter.

The molten salt can be selected from chloride molten salts and fluoridemolten salts. A molten salt mixture containing a chloride molten saltand a fluoride molten salt may be used.

Examples of chloride molten salts include KCl, NaCl, CaCl₂, LiCl, RbCl,CsCl, SrCl₂, BaCl₂, and MgCl₂. Examples of fluoride molten salts includeLiF, NaF, KF, RbF, CsF, MgF₂, CaF₂, SrF₂, and BaF₂. Chloride moltensalts are preferably used in view of efficiency; in particular, KCl,NaCl, and CaCl₂ are preferably used because they are inexpensive andeasily available.

Among such molten salts, a plurality of molten salts can be combined andused as a molten salt having a desirable composition. For example, amolten salt having a composition such as KCl—CaCl₂, LiCl—KCl, orNaCl—KCl may be used.

In a method for producing a metal by molten salt electrolysis accordingto this embodiment, the following apparatuses can be preferably used.That is, preferably, the apparatus includes a container containing amolten salt; a cathode immersed in the molten salt contained within thecontainer; and an anode that is immersed in the molten salt containedwithin the container and that contains a conductive treatment objectcontaining two or more metal elements, wherein the molten salt ismovable into and out of the anode, the apparatus further includes acontrol unit configured to control the potential of the cathode and theanode to a predetermined value, and the value of the potential ischangeable in the control unit. An apparatus used for a method forproducing a metal by molten salt electrolysis according to thisembodiment is preferably an apparatus that includes a containercontaining a molten salt containing a dissolved particular metal; and acathode and an anode that are immersed in the molten salt containedwithin the container, wherein the apparatus includes a control unitconfigured to control the potential of the cathode and the anode to apredetermined value, and the value of the potential is changeable in thecontrol unit.

The apparatuses will be described with reference to FIGS. 18 and 19. Anapparatus illustrated in FIG. 18 includes a container 1 containing amolten salt, a molten salt 2 contained within the container 1, a basket4 containing a treatment object 3 containing two or more metal elements,an electrode 6, a heater 10 for heating the molten salt 2, and a controlunit 9 electrically connected to the basket 4 and the electrode 6 viaconductive wires 5.

The control unit 9 is configured to control the potential of oneelectrode (anode) that is the basket 4 and the other electrode (cathode)that is the electrode 6, to a predetermined value. In the control unit9, the value to which the potential is controlled is changeable. Theheater 10 is disposed so as to circularly surround the container 1. Theelectrode 6 may be formed of a desired material, for example, carbon.The container 1 may have a bottom surface that has a circular shape or apolygonal shape. The basket 4 may be the above-described basket.

The potential of the basket 4 and the electrode 6 is controlled by thecontrol unit 9 to a predetermined potential value. As a result, aparticular metal is dissolved in the molten salt 2 from the treatmentobject 3.

After the particular metal is sufficiently dissolved from the treatmentobject 3, the basket 4 and the electrode 6 are removed and anotherelectrode 7 (cathode) and another electrode 8 (anode) are placed in themolten salt 2. These electrodes 7 and 8 are connected to the controlunit 9 via conductive wires 5. The control unit 9 is used to control thepotential of the electrodes 7 and 8 to a predetermined value. At thistime, the potential is controlled such that the potential at theelectrode 7 is the deposition potential of the particular metal. As aresult, the particular metal dissolved in the molten salt 2 is depositedon the surface of the electrode 7 (cathode). The electrodes 7 and 8 maybe formed of a material such as glassy carbon (C).

The heating temperature for the molten salt 2 with the heater 10 may be,for example, 800° C. in both of the treatments using the apparatusesillustrated in FIGS. 18 and 19. In this way, the particular metal can bedeposited as a simple substance on the surface of the electrode 7.

The potential of the electrodes 7 and 8 may be controlled such that analloy of the particular metal and the cathode material is deposited onthe surface of the electrode 7 (cathode). In this case, theabove-described dissolution step and deposition step may be performedwith the alloyed electrode 7. That is, the apparatus illustrated in FIG.18 is newly prepared and the electrode 7 alloyed with the particularmetal is used instead of the above-described treatment object 3.

In the cases where the method for producing a metal of this embodimentis performed with the apparatuses illustrated in FIGS. 18 and 19, forexample, the method may be performed in the following manner.Hereinafter, examples relating to vanadium, molybdenum, strontium, andgermanium will be described.

(Vanadium)

For example, the method for producing a metal of this embodiment is usedto obtain vanadium. Ferrovanadium (1 kg) is first prepared as thetreatment object 3 and NaCl—KCl is prepared as the molten salt 2. Forexample, the ferrovanadium contains 75 wt % of vanadium (V) and 25 wt %of iron (Fe). The ferrovanadium is ground and placed within the basket4. The amount of the molten salt 2 is about 15 liters.

The above-described dissolution step may be performed with a carbonelectrode serving as the electrode 6. Subsequently, the deposition stepmay be performed with electrodes formed of glassy carbon and serving asthe electrodes 7 and 8.

(Molybdenum)

The method for producing a metal of this embodiment is used to obtainmolybdenum. Mo—Cu heat spreaders (1 kg) are first prepared as thetreatment object 3 and LiCl—KCl is prepared as the molten salt 2. Forexample, the Mo—Cu heat spreaders contain 50 wt % of molybdenum (Mo) and50 wt % of copper (Cu). The heat spreaders are ground and placed withinthe basket 4. The amount of the molten salt 2 is about 5 liters.

The above-described dissolution step may be performed with a carbonelectrode serving as the electrode 6. Subsequently, the deposition stepmay be performed with electrodes formed of glassy carbon and serving asthe electrodes 7 and 8.

(Strontium)

The method for producing a metal of this embodiment is used to obtainmolybdenum. An oxide superconducting material (1 kg) is first preparedas the treatment object 3 and LiF—CaF₂ is prepared as the molten salt 2.For example, the oxide superconducting material contains 17 wt % ofstrontium (Sr) and 8 wt % of calcium (Ca). The oxide superconductingmaterial is ground and placed within the basket 4. The amount of themolten salt 2 is about 4 liters.

The above-described dissolution step may be performed with a carbonelectrode serving as the electrode 6. Subsequently, the deposition stepmay be performed with electrodes formed of glassy carbon and serving asthe electrodes 7 and 8.

(Germanium)

The method for producing a metal of this embodiment is used to obtaingermanium. An optical fiber material (1 kg) is first prepared as thetreatment object 3 and LiF—CaF₂ is prepared as the molten salt 2. Forexample, the optical fiber material contains 3 wt % of germanium (Ge).The optical fiber material is ground and placed within the basket 4. Theamount of the molten salt 2 is about 4 liters.

The above-described dissolution step may be performed with a carbonelectrode serving as the electrode 6. Subsequently, the deposition stepmay be performed with electrodes formed of glassy carbon and serving asthe electrodes 7 and 8.

As has been described, by using ferrovanadium, Mo—Cu heat spreaders,oxide superconducting material, and optical fiber material as thetreatment object 3, vanadium, molybdenum, strontium, and germanium canbe obtained, respectively. From the viewpoint of enhancement of thetreatment efficiency, the size of ferrovanadium, Mo—Cu heat spreaders,oxide superconducting material, and optical fiber material serving asthe treatment object 3 is preferably minimized by grinding: for example,the treatment object 3 is preferably ground into particles having amaximum particle size of 5 mm or less, more preferably 3 mm or less,still more preferably 1 mm or less.

According to the method for producing a metal by molten saltelectrolysis of this embodiment, the apparatus configuration can besimplified and the treatment time can also be decreased, compared withthe existing recovery methods and the like. Thus, the cost incurred canbe reduced. In addition, by appropriately setting a potential at anelectrode, a particular metal can be deposited as a simple substance onthe electrode surface and hence high purity metal can be obtained.

The potentials for depositing vanadium, a vanadium alloy, molybdenum, amolybdenum alloy, strontium, a strontium alloy, germanium, and agermanium alloy can be determined by the above-described calculation.

First to Fourth embodiments have been individually described so far.However, for example, in order to obtain tungsten, lithium, transitionmetals, and rare earth metals in Second to Fourth embodiments, methodsin other embodiments may be entirely or partially employed.

EXAMPLES First Embodiment Example

Nd, Dy, and Pr were produced by molten salt electrolysis from an orecontaining rare earth metals.

(Sample)

The ore serving as a treatment object was xenotime ore. The xenotime orewas ground with a crusher or a ball mill so as to have a particle sizeof about 2 mm. The ground sample (xenotime ore) was wrapped with amolybdenum (Mo) mesh (50 mesh).

As illustrated in FIG. 14, the sample powder contained within the meshwas used as an anode (anode electrode).

(Details of Experiment)

A molten LiF—NaF—KF eutectic salt was employed as the molten salt. Thissalt was completely melted by heating at 700° C. In this molten salt,the above-described anode electrode and a cathode electrode were wiredand immersed. The cathode electrode was formed of glassy carbon.

Dissolution Step:

While the anode electrode and the cathode electrode were thus immersedin the molten salt, the anode electrode was maintained at apredetermined potential. After about 4 hours lapsed, a sample was takenfrom the molten salt and the sample was subjected to compositionanalysis by inductively coupled plasma-atomic emission spectroscopy(ICP-AES).

Electrolysis Step:

After the dissolution step, a cathode electrode formed of Ni and ananode electrode formed of glassy carbon were immersed in the moltensalt. The potential at the cathode electrode was maintained at apredetermined potential. Specifically, the potential was maintained suchthat Dy—Ni alloy was formed in the LiF—NaF—KF molten salt. After apredetermined time lapsed, the surface status of the cathode electrodewas observed.

(Result) Regarding Dissolution Step:

The anode current observed in the dissolution step varied with time asillustrated in FIG. 15.

In FIG. 15, the abscissa axis indicates time (unit: min), and theordinate axis indicates the value of anode current (unit: mA). Asillustrated in FIG. 15, the current value decreased with time. Thechange rate of current value with respect to time had the followingtendency: the change rate was the highest at the beginning of themeasurement (at the beginning of application of current) and, afterthat, the change rate gradually decreased.

The sample taken from the molten salt was subjected to compositionanalysis by ICP-AES. As a result, dissolution of Nd and Dy in the moltensalt was confirmed.

Regarding Electrolysis Step:

FIGS. 16 and 17 illustrate results of observation of a section of thesurface layer of the cathode electrode with a scanning electronmicroscope (SEM). As illustrated in FIGS. 16 and 17, Dy—Ni alloy 32 wasdeposited on the surface of an electrode body part 31 constituting thecathode electrode and formed of Ni. This Dy—Ni alloy 32 was probablyformed by the reaction between Dy present in the molten salt and Niconstituting the cathode electrode, and deposited on the surface of thecathode electrode. In this way, Dy contained in the xenotime ore can beseparated and extracted as Dy—Ni alloy from the ore.

FIG. 16 illustrates a back-scattered electron image observed with theSEM. FIG. 17 illustrates the distribution of Dy atoms in the regionsillustrated in FIG. 16 and subjected to X-ray analysis. As illustratedin FIG. 17, Dy was scarcely detected in a region 33 corresponding to theelectrode body part 31; however, Dy was detected in a region 34corresponding to the Dy—Ni alloy 32.

Second Embodiment Example

Cemented carbide tools were used as the metal material containingtungsten and tungsten was produced by molten salt electrolysis.

(Sample)

The cemented carbide tools serving as a treatment object were cuttingtools containing 90 wt % of tungsten carbide and 10 wt % of cobaltserving as a binder. The cutting tools were ground with a bead mill oran attritor so as to have a particle size of about 2 mm. The groundsample (cutting tools) was wrapped with a molybdenum (Mo) mesh (50mesh). As illustrated in FIG. 14, the sample powder (treatment object)contained within the Mo mesh was used as an anode (anode electrode).

(Details of Experiment)

A molten NaCl—KCl eutectic salt was employed as the molten salt. Thissalt was completely melted by heating at 700° C. In this molten salt,the above-described anode electrode and a cathode electrode were wiredand immersed. The cathode electrode was formed of glassy carbon.

Dissolution Step:

While the anode electrode and the cathode electrode were thus immersedin the molten salt, the anode electrode was maintained at apredetermined potential. After a predetermined time lapsed, a sample wastaken from the molten salt and the sample was subjected to compositionanalysis by ICP-AES.

Electrolysis Step:

After the dissolution step, a cathode electrode formed of glassy carbonand an anode electrode formed of glassy carbon were immersed in themolten salt. The potential at the cathode electrode was maintained at apredetermined potential. Specifically, the potential was maintained suchthat tungsten was deposited in the NaCl—KCl molten salt. After apredetermined time lapsed, the surface status of the cathode electrodewas observed.

(Result) Regarding Dissolution Step:

The anode current observed in the dissolution step varied with time asin First embodiment (example) (FIG. 15). In FIG. 15, the abscissa axisindicates time (unit: min), and the ordinate axis indicates the value ofanode current (unit: mA). As illustrated in FIG. 15, the current valuedecreased with time. The change rate of current value with respect totime had the following tendency: the change rate was the highest at thebeginning of the measurement (at the beginning of application ofcurrent) and, after that, the change rate gradually decreased.

The sample taken from the molten salt was subjected to compositionanalysis by ICP-AES. As a result, dissolution of tungsten in the moltensalt was confirmed.

Regarding Electrolysis (Deposition) Step:

Observation of a section of the surface layer of the cathode electrodewith a scanning electron microscope (SEM) revealed deposition oftungsten on the surface of an electrode body part constituting thecathode electrode and formed of glassy carbon.

In this way, high purity tungsten was obtained from the cemented carbidecutting tools containing tungsten.

Third Embodiment Example

Commercially available lithium-ion secondary batteries were used as thetreatment object containing lithium and lithium was produced by moltensalt electrolysis.

(Sample)

Commercially available lithium-ion secondary batteries (the positiveelectrode was formed of lithium cobalt oxide and the negative electrodewas formed of graphite, lithium cobalt oxide content: mass %)

(Separation of Lithium Battery Positive Electrode Material)

The lithium-ion secondary batteries were immersed in an electrolyticsolution (5% NaCl) and discharged until the voltage became 0.1 mV. Afterthat, the positive electrode material was taken out by manualdisassembly, and ground with a cutter mill to provide a positiveelectrode material powder having an average particle size of 0.1 mm. Thecomposition of the powder is described in Table I. As a result ofanalysis, it was confirmed that the powder obtained by the separationwas lithium cobalt oxide.

TABLE I Composition (mass %) Li 7 Co 60

The powder was wrapped with a molybdenum (Mo) mesh (200 mesh). Asillustrated in FIG. 14, the sample powder contained within the Mo meshwas used as an anode (anode electrode).

(Preparation of Electrolysis Apparatus)

A molten NaCl—KCl eutectic salt was employed as the molten salt. Thissalt was completely melted by heating at 700° C. In this molten salt,the above-described anode electrode and a cathode electrode were wiredand immersed. The cathode (cathode electrode) was formed of carbon.

(Electrolysis Dissolution Step)

While the anode electrode and the cathode electrode were thus immersedin the molten salt, the anode electrode was maintained at apredetermined potential. After a predetermined time lapsed, a sample wastaken from the molten salt and the sample was subjected to compositionanalysis by ICP-AES.

The anode current observed in the dissolution step varied with time asin First embodiment (example) (FIG. 15). In FIG. 15, the abscissa axisindicates time (unit: min), and the ordinate axis indicates the value ofanode current (unit: mA). As illustrated in FIG. 15, the current valuedecreased with time. The change rate of current value with respect totime had the following tendency: the change rate was the highest at thebeginning of the measurement (at the beginning of application ofcurrent) and, after that, the change rate gradually decreased.

The sample taken from the molten salt was subjected to compositionanalysis by ICP-AES. As a result, dissolution of lithium in the moltensalt was confirmed.

(Electrolysis Deposition Step)

After the dissolution step, a cathode electrode formed of glassy carbonand an anode electrode formed of glassy carbon were immersed in themolten salt. The potential at the cathode electrode was maintained at apredetermined potential. Specifically, the potential was maintained suchthat lithium was deposited in the NaCl—KCl molten salt. After apredetermined time lapsed, a section of the surface layer of the cathodeelectrode was observed with a scanning electron microscope (SEM).

The observation revealed deposition of lithium on the surface of anelectrode body part constituting the cathode electrode and formed ofglassy carbon.

In this way, lithium was recovered from the positive electrode materialcontaining lithium.

Fourth Embodiment (Example)-1

Ferrovanadium was used as the metal material containing vanadium andvanadium was produced by molten salt electrolysis.

(Sample)

The ferrovanadium serving as a treatment object contained 75 wt % ofvanadium and 25 wt % of iron. The ferrovanadium was ground with a beadmill or an attritor so as to have a particle size of about 2 mm. Theground sample (ferrovanadium) was wrapped with a molybdenum (Mo) mesh(50 mesh). As illustrated in FIG. 14, the sample powder (treatmentobject) contained within the Mo mesh was used as an anode (anodeelectrode).

(Details of Experiment)

A molten NaCl—KCl eutectic salt was employed as the molten salt. Thissalt was completely melted by heating at 700° C. In this molten salt,the above-described anode electrode and a cathode electrode were wiredand immersed. The cathode electrode was formed of glassy carbon.

Dissolution Step:

While the anode electrode and the cathode electrode were thus immersedin the molten salt, the anode electrode was maintained at apredetermined potential. At this time, the potential was set such thatiron was not dissolved but vanadium alone was selectively dissolved.After a predetermined time lapsed, a sample was taken from the moltensalt and the sample was subjected to composition analysis by ICP-AES.

Electrolysis Step:

After the dissolution step, a cathode electrode formed of glassy carbonand an anode electrode formed of glassy carbon were immersed in themolten salt. The potential at the cathode electrode was maintained at apredetermined potential. Specifically, the potential was maintained suchthat vanadium was deposited in the NaCl—KCl molten salt. After apredetermined time lapsed, the surface status of the cathode electrodewas observed.

(Result) Regarding Dissolution Step:

The anode current observed in the dissolution step varied with time asin First embodiment (example) (FIG. 15). In FIG. 15, the abscissa axisindicates time (unit: min), and the ordinate axis indicates the value ofanode current. As illustrated in FIG. 15, the current value decreasedwith time. The change rate of current value with respect to time had thefollowing tendency: the change rate was the highest at the beginning ofthe measurement (at the beginning of application of current) and, afterthat, the change rate gradually decreased.

The sample taken from the molten salt was subjected to compositionanalysis by ICP-AES. As a result, dissolution of vanadium in the moltensalt was confirmed.

Regarding Electrolysis (Deposition) Step:

Observation of a section of the surface layer of the cathode electrodewith a scanning electron microscope (SEM) revealed deposition ofvanadium on the surface of an electrode body part constituting thecathode electrode and formed of glassy carbon.

In this way, high purity vanadium was obtained from the ferrovanadiumcontaining vanadium.

Fourth Embodiment (Example)-2

Mo—Cu heat spreaders were used as the metal material containingmolybdenum and molybdenum was produced by molten salt electrolysis.

(Sample)

The Mo—Cu heat spreaders serving as a treatment object contained 50 wt %of molybdenum and 50 wt % of copper. The heat spreaders were ground witha bead mill or an attritor so as to have a particle size of about 2 mm.The ground sample (heat spreaders) was wrapped with a platinum (Pt) mesh(50 mesh). The sample powder (treatment object) contained within the Ptmesh was used as an anode (anode electrode).

(Details of Experiment)

A molten LiCl—KCl eutectic salt was employed as the molten salt. Thissalt was completely melted by heating at 450° C. In this molten salt,the above-described anode electrode and a cathode electrode were wiredand immersed. The cathode electrode was formed of glassy carbon.

Dissolution Step:

While the anode electrode and the cathode electrode were thus immersedin the molten salt, the anode electrode was maintained at apredetermined potential. At this time, the potential was set such thatcopper was not dissolved but molybdenum alone was selectively dissolved.After a predetermined time lapsed, a sample was taken from the moltensalt and the sample was subjected to composition analysis by ICP-AES.

Electrolysis Step:

After the dissolution step, a cathode electrode formed of glassy carbonand an anode electrode formed of glassy carbon were immersed in themolten salt. The potential at the cathode electrode was maintained at apredetermined potential. Specifically, the potential was maintained suchthat molybdenum was deposited in the LiCl—KCl molten salt. After apredetermined time lapsed, the surface status of the cathode electrodewas observed.

(Result) Regarding Dissolution Step:

The value of anode current observed in the dissolution step decreasedwith time as in the above-described case relating to vanadium. Thechange rate of current value with respect to time had the followingtendency: the change rate was the highest at the beginning of themeasurement (at the beginning of application of current) and, afterthat, the change rate gradually decreased.

The sample taken from the molten salt was subjected to compositionanalysis by ICP-AES. As a result, dissolution of molybdenum in themolten salt was confirmed.

Regarding Electrolysis (Deposition) Step:

Observation of a section of the surface layer of the cathode electrodewith a scanning electron microscope (SEM) revealed deposition ofmolybdenum on the surface of an electrode body part constituting thecathode electrode and formed of glassy carbon.

In this way, high purity molybdenum was obtained from the heat spreaderscontaining molybdenum.

Fourth Embodiment (Example)-3

An oxide superconducting material was used as the metal materialcontaining strontium and strontium was produced by molten saltelectrolysis.

(Sample)

The oxide superconducting material serving as a treatment objectcontained 17 wt % of strontium and 8 wt % of calcium. The oxidesuperconducting material was ground with a bead mill or an attritor soas to have a particle size of about 2 mm. The ground sample (oxidesuperconducting material) was wrapped with a platinum (Pt) mesh (50mesh). The sample powder (treatment object) contained within the Pt meshwas used as an anode (anode electrode).

(Details of Experiment)

A molten LiF—CaF₂ eutectic salt was employed as the molten salt. Thissalt was completely melted by heating at 850° C. In this molten salt,the above-described anode electrode and a cathode electrode were wiredand immersed. The cathode electrode was formed of glassy carbon.

Dissolution Step:

While the anode electrode and the cathode electrode were thus immersedin the molten salt, the anode electrode was maintained at apredetermined potential. At this time, the potential was set such thatstrontium and calcium alone were selectively dissolved and the otherelements contained were not dissolved. After a predetermined timelapsed, a sample was taken from the molten salt and the sample wassubjected to composition analysis by ICP-AES.

Electrolysis Step:

After the dissolution step, a cathode electrode formed of glassy carbonand an anode electrode formed of glassy carbon were immersed in themolten salt. The potential at the cathode electrode was maintained at apredetermined potential. Specifically, the potential was maintained suchthat strontium was deposited in the LiF—CaF₂ molten salt. After apredetermined time lapsed, the surface status of the cathode electrodewas observed.

(Result) Regarding Dissolution Step:

The value of anode current observed in the dissolution step decreasedwith time as in the above-described case relating to vanadium. Thechange rate of current value with respect to time had the followingtendency: the change rate was the highest at the beginning of themeasurement (at the beginning of application of current) and, afterthat, the change rate gradually decreased.

The sample taken from the molten salt was subjected to compositionanalysis by ICP-AES. As a result, dissolution of strontium in the moltensalt was confirmed.

Regarding Electrolysis (Deposition) Step:

Observation of a section of the surface layer of the cathode electrodewith a scanning electron microscope (SEM) revealed adhesion of strontiumto the surface of an electrode body part constituting the cathodeelectrode and formed of glassy carbon. Since strontium has a meltingpoint of 768° C., strontium was in the liquid state. When the amount ofstrontium adhering to the electrode body becomes large, the strontiumrises to the surface due to the specific gravity difference relative tothe molten salt. Accordingly, a jig for collecting strontium rising tothe surface was disposed on the upper side of the electrode.

In this way, high purity strontium was obtained from the oxidesuperconducting material containing strontium.

Fourth Embodiment (Example)-4

An optical fiber material was used as the metal material containinggermanium and germanium was produced by molten salt electrolysis.

(Sample)

The optical fiber material serving as a treatment object contained 3 wt% of germanium. The optical fiber material was ground with a bead millor an attritor so as to have a particle size of about 2 mm. The groundsample (optical fiber material) was wrapped with a platinum (Pt) mesh(50 mesh). The sample powder (treatment object) contained within the Ptmesh was used as an anode (anode electrode).

(Details of Experiment)

A molten LiF—CaF₂ eutectic salt was employed as the molten salt. Thissalt was completely melted by heating at 850° C. In this molten salt,the above-described anode electrode and a cathode electrode were wiredand immersed. The cathode electrode was formed of glassy carbon.

Dissolution Step:

While the anode electrode and the cathode electrode were thus immersedin the molten salt, the anode electrode was maintained at apredetermined potential. At this time, the potential was set such thatgermanium alone was selectively dissolved and the other elementscontained were not dissolved. After a predetermined time lapsed, asample was taken from the molten salt and the sample was subjected tocomposition analysis by ICP-AES.

Electrolysis Step:

After the dissolution step, a cathode electrode formed of glassy carbonand an anode electrode formed of glassy carbon were immersed in themolten salt. The potential at the cathode electrode was maintained at apredetermined potential. Specifically, the potential was maintained suchthat germanium was deposited in the LiF—CaF₂ molten salt. After apredetermined time lapsed, the surface status of the cathode electrodewas observed.

(Result) Regarding Dissolution Step:

The anode current observed in the dissolution step decreased with timeas in the above-described case relating to vanadium. The change rate ofcurrent value with respect to time had the following tendency: thechange rate was the highest at the beginning of the measurement (at thebeginning of application of current) and, after that, the change rategradually decreased.

The sample taken from the molten salt was subjected to compositionanalysis by ICP-AES. As a result, dissolution of germanium in the moltensalt was confirmed.

Regarding Electrolysis (Deposition) Step:

Observation of a section of the surface layer of the cathode electrodewith a scanning electron microscope (SEM) revealed deposition ofgermanium on the surface of an electrode body part constituting thecathode electrode and formed of glassy carbon.

In this way, high purity germanium was obtained from the optical fibermaterial containing germanium.

The embodiments and examples disclosed above are given by way ofillustration in all respects and should be considered as non-limitative.The scope of the present invention is indicated not by the abovedescriptions but by Claims and is intended to embrace all themodifications within the meaning and range of equivalency of the Claims.

INDUSTRIAL APPLICABILITY

The present invention is suitably applicable to a method for obtaining aparticular metal at high purity from a treatment object containing twoor more metal elements. The present invention is also suitablyapplicable to a method for obtaining a desirable metal from an ore or acrude metal ingot. The present invention is also suitably applicable toa method for obtaining tungsten at high purity from a treatment objectcontaining at least one of tungsten and lithium.

REFERENCE SIGNS LIST

1 container; 2 molten salt; 3 treatment object; 4, 24 basket; 5conductive wire; 6 to 8, 15, 27 electrode; 9 control unit; 10 heater; 11DyNi₂ film; 12 Pr film; 13 Nd film; 16 Dy film; 25 electrode material;26 alloy; 31 electrode body part; 32 Dy—Ni alloy; 33, 34 region

1. A method for producing a metal by molten salt electrolysis, themethod comprising: a step of dissolving, in a molten salt, a metalelement contained in a treatment object containing two or more metalelements; and a step of depositing or alloying a particular metalpresent in the molten salt, on one of a pair of electrode membersdisposed in the molten salt containing the dissolved metal element, bycontrolling a potential of the electrode members to a predeterminedvalue.
 2. The method for producing a metal by molten salt electrolysisaccording to claim 1, wherein the treatment object is an ore or a crudemetal ingot obtained from the ore.
 3. The method for producing a metalby molten salt electrolysis according to claim 1, wherein the method isa method for producing tungsten, a metal element contained in thetreatment object is tungsten, in the step of dissolving, in a moltensalt, a metal element from a treatment object, tungsten is dissolvedfrom the treatment object, and in the step of depositing or alloying aparticular metal, tungsten present in the molten salt is deposited onone of a pair of electrode members disposed in the molten saltcontaining dissolved tungsten, by controlling a potential of theelectrode members to a predetermined value.
 4. The method for producinga metal by molten salt electrolysis according to claim 3, wherein thetreatment object is a metal material containing the tungsten.
 5. Themethod for producing tungsten a metal by molten salt electrolysisaccording to claim 3, wherein the treatment object is a metal materialcontaining tungsten and a transition metal.
 6. The method for producinga metal by molten salt electrolysis according to claim 3, wherein thetreatment object is a cemented carbide product.
 7. The method forproducing a metal by molten salt electrolysis according to claim 1,wherein the method is a method for producing lithium, a metal elementcontained in the treatment object is lithium, in the step of dissolving,in a molten salt, a metal element from a treatment object, lithium isdissolved from the treatment object, and in the step of depositing oralloying a particular metal, lithium present in the molten salt isdeposited on one of a pair of electrode members disposed in the moltensalt containing dissolved lithium, by controlling a potential of theelectrode members to a predetermined value.
 8. The method for producinga metal by molten salt electrolysis according to claim 7, wherein thetreatment object is a material containing lithium and a transitionmetal.
 9. The method for producing a metal by molten salt electrolysisaccording to claim 7, wherein the treatment object is a batteryelectrode material containing lithium.
 10. The method for producing ametal by molten salt electrolysis according to claim 1, wherein thetreatment object contains a transition metal or a rare earth metal. 11.The method for producing a metal by molten salt electrolysis accordingto claim 1, wherein the treatment object contains one or more metalsselected from the group consisting of V, Nb, Mo, Ti, Ta, Zr, and Hf. 12.The method for producing a metal by molten salt electrolysis accordingto claim 1, wherein the treatment object contains Sr and/or Ba.
 13. Themethod for producing a metal by molten salt electrolysis according toclaim 1, wherein the treatment object contains one or more metalsselected from the group consisting of Zn, Cd, Ga, In, Ge, Sn, Pb, Sb,and Bi.
 14. The method for producing a metal by molten salt electrolysisaccording to claim 1, wherein the molten salt is selected such that, inthe step of depositing or alloying a particular metal, a differencebetween a standard electrode potential of a simple substance or alloy ofthe particular metal and a standard electrode potential of a simplesubstance or alloy of another metal in the molten salt is 0.05 V ormore.
 15. The method for producing a metal by molten salt electrolysisaccording to claim 1, wherein, in the step of depositing or alloying aparticular metal, the potential of the electrode members is controlledto the predetermined value so that the particular metal in the moltensalt is selectively deposited or alloyed.
 16. The method for producing ametal by molten salt electrolysis according to claim 1, wherein, in thestep of dissolving, in a molten salt, a metal element contained in atreatment object, the metal element is dissolved in the molten salt by achemical procedure.
 17. The method for producing a metal by molten saltelectrolysis according to claim 1, wherein, in the step of dissolving,in a molten salt, a metal element contained in a treatment object, acathode and an anode that is formed of an anode material containing thetreatment object are disposed in the molten salt, and a potential at theanode is controlled to a predetermined value so that a metal elementcorresponding to the controlled potential is dissolved in the moltensalt from the treatment object.
 18. The method for producing a metal bymolten salt electrolysis according to claim 17, wherein the molten saltis selected such that, in the step of dissolving, in a molten salt, ametal element contained in a treatment object, a difference between astandard electrode potential of a simple substance or alloy of theparticular metal and a standard electrode potential of a simplesubstance or alloy of another metal in the molten salt is 0.05 V ormore.
 19. The method for producing a metal by molten salt electrolysisaccording to claim 17, wherein, in the step of dissolving, in a moltensalt, a metal element contained in a treatment object, the potential atthe anode is controlled to a predetermined value so that the particularmetal element is selectively dissolved in the molten salt.
 20. Themethod for producing a metal by molten salt electrolysis according toclaim 1, wherein, in the step of dissolving, in a molten salt, a metalelement contained in a treatment object, one or more metals each servingas the particular metal are dissolved in the molten salt.
 21. The methodfor producing a metal by molten salt electrolysis according to claim 1,wherein the particular metal deposited or alloyed is a transition metal.22. The method for producing a metal by molten salt electrolysisaccording to claim 1, wherein the particular metal deposited or alloyedis a rare earth metal.
 23. The method for producing a metal by moltensalt electrolysis according to claim 1, wherein the particular metaldeposited or alloyed is V, Nb, Mo, Ti, Ta, Zr, or Hf.
 24. The method forproducing a metal by molten salt electrolysis according to claim 1,wherein the particular metal deposited or alloyed is Sr or Ba.
 25. Themethod for producing a metal by molten salt electrolysis according toclaim 1, wherein the particular metal deposited or alloyed is Zn, Cd,Ga, In, Ge, Sn, Pb, Sb, or Bi.
 26. The method for producing a metal bymolten salt electrolysis according to claim 1, wherein the molten saltis a chloride or fluoride molten salt.
 27. The method for producing ametal by molten salt electrolysis according to claim 1, wherein themolten salt is a molten salt mixture containing a chloride molten saltand a fluoride molten salt.
 28. The method for producing a metal bymolten salt electrolysis according to claim 1, wherein the treatmentobject has a form of particles or powder.
 29. The method for producing ametal by molten salt electrolysis according to claim 28, wherein thetreatment object having the form of particles or powder is compacted toform the anode.
 30. A method for producing a metal by molten saltelectrolysis, the method being a method for producing a particular metalby molten salt electrolysis from a treatment object containing two ormore metal elements, wherein a cathode and an anode that is formed of ananode material containing the treatment object are disposed in a moltensalt, and a potential at the anode is controlled to a predeterminedvalue so that a particular metal is left in the anode by performing astep of dissolving a metal element corresponding to the controlledpotential in the molten salt from the treatment object.
 31. The methodfor producing a metal by molten salt electrolysis according to claim 30,wherein the treatment object is an ore or a crude metal ingot obtainedfrom the ore.
 32. The method for producing a metal by molten saltelectrolysis according to claim 30, wherein the method is a method forproducing tungsten by molten salt electrolysis from a treatment objectcontaining tungsten, a cathode and an anode that is formed of an anodematerial containing the treatment object are disposed in a molten salt,and a potential at the anode is controlled to a predetermined value sothat a metal element corresponding to the controlled potential isdissolved in the molten salt from the treatment object and tungsten isleft in the anode.
 33. The method for producing a metal by molten saltelectrolysis according to claim 30, wherein the molten salt is selectedsuch that, in the step of dissolving a metal element in the molten saltfrom the treatment object, a difference between a standard electrodepotential of a simple substance or alloy of the particular metal and astandard electrode potential of a simple substance or alloy of anothermetal in the molten salt is 0.05 V or more.
 34. An apparatus used for amethod for producing a metal by molten salt electrolysis, the apparatuscomprising: a container containing a molten salt; a cathode immersed inthe molten salt contained within the container; and an anode that isimmersed in the molten salt contained within the container and thatcontains a treatment object containing two or more metal elements,wherein the molten salt is movable into and out of the anode, theapparatus further comprises a control unit configured to control apotential of the cathode and the anode to a predetermined value, and avalue of the potential is changeable in the control unit.
 35. Anapparatus used for a method for producing a metal by molten saltelectrolysis, the apparatus comprising: a container containing a moltensalt containing two or more dissolved metal elements; a cathode and ananode that are immersed in the molten salt contained within thecontainer; and a control unit configured to control a potential of thecathode and the anode to a predetermined value, wherein a value of thepotential is changeable in the control unit.
 36. The apparatus accordingto claim 34, wherein the two or more metal elements include at least oneof tungsten and lithium.
 37. The apparatus according to claim 35,wherein the two or more metal elements include at least one of tungstenand lithium.